STORAGE   BATTERIES 


THE  MACMILLAN  COMPANY 

NEW  YORK    •    BOSTON    •    CHICAGO 
DALLAS   •    SAN    FRANCISCO 

MACMILLAN   &  CO.,  LIMITED 

LONDON   •    BOMBAY   •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


STORAGE  BATTERIES 


THE  CHEMISTRY  AND  PHYSICS  OF  THE 
LEAD   ACCUMULATOR 


BY 

HARRY  W.    MORSE,  PH.D. 

ASSISTANT    PROFESSOR    OF    PHYSICS 
IN    HARVARD    UNIVERSITY 


gorfe 

THE   MACMILLAN   COMPANY 
1912 

All  rights  reserved 


Ml, 

Engineering 
Library 


COPTBIGHT,    1912, 

BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  February,  1912. 


J.  8.  Cashing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


CONTENTS 

CHAPTER  PAGE 

I.  INTRODUCTORY  AND  HISTORICAL       ...  1 

II.  SOME  ELECTROCHEMICAL  FUNDAMENTALS        .  10 

III.  ABOUT  IONS 30 

IV.  THE  FUNDAMENTAL  CELL-REACTION         .        .  39 
V.     THE  ACTIVE  IONS 47 

VI.  SOME  PERTINENT  PHYSICAL  QUERIES       .        .  56 

VII.     ENERGY  RELATIONS 64 

VIII.  REACTIONS  AT  THE  ELECTRODES       ...  80 

IX.     CHARGE  AND  DISCHARGE 94 

X.     CAPACITY 116 

XI.     EFFICIENCY 141 

XII.  INTERNAL  RESISTANCE        ...        .        .  148 

XIII.  PHYSICAL  CHARACTERISTICS       ....  172 

XIV.  FORMATION  OF  PLANTE  PLATES        .        .        .  179 
XV.  PASTE  PLATES     .        ...        .        .        .194 

XVI.  DISEASES  AND  TROUBLES    ...        .        .  205 

XVII.  SOME  COMMERCIAL  TYPES                 ,,        .        .  225 

XVIII.  ACCUMULATORS  IN  GENERAL     ....  246 

APPENDIX    .        .        .        .        .        .        .        .        .        .  255 


238199 


STORAGE  BATTERIES 

CHAPTER   I 
INTRODUCTORY  AND  HISTORICAL 

1.  Into  our  present  age  of  power,  where  we  reckon 
by  thousands  and  tens  of  thousands  of  kilowatts, 
there  has  come  down  from  a  previous  era  one  single 
form  of  the  galvanic  cell  which  retains  sufficient 
commercial  importance  to  be  worth  consideration  in 
connection  with  modern  power  plants  and  modern 
power  operation.  This  is  the  lead-sulphuric  acid 
accumulator.  It  was  invented  and  perfected  in  the 
heyday  of  galvanic  cells  —  at  a  time  before  the  dy- 
namo and  the  electric  motor  had  any  technical  im- 
portance. In  our  own  laboratory,  hidden  away  in 
the  attic  where  cast-off  things  are  stored,  lie  the 
remains  of  the  big  Bunsen  cells  which  were  once  the 
source  of  our  heaviest  currents  and  with  which  the 
remarkable  phenomena  of  current  electricity  were 
shown  to  classes  and  in  public  lectures  in  those 
days.  These  same  cells  were  used  to  charge  small 
storage  cells  of  the  original  Plante  type  —  mere  strips 


STORAGE.  BATTERIES 


of  lead,  separated  by  soft  rubber  insulators  arid  rolled 
into  spiral  form  ;  then  formed  with  the  aid  of  the 
primary  cells,  by  a  series  of  reversals,  until  the  plates 
attained  a  certain  capacity.  One  of  these  cells  is 
shown  in  Figure  1.  With  these  storage  cells,  which 
have  low  resistance  and  high  current-giving  capacity 
even  in  comparison  with  the  large  Bunsen  cells,  the 
most  wonderful  experiments  could  be 
performed  —  experiments  which  are  to 
us  now  so  commonplace  and  so  much  a 
part  of  our  everyday  life  that  their  de- 
scription brings  a  smile  from  the  high- 
school  boy  who  has  studied  physics  and 
chemistry.  These  cells  would  run  an 
arc  light  for  several  minutes;  heat 
small  platinum  wires  to  the  melting 
point ;  provide  current  for  electro- 
magnets of  power  enormous  for  that 
FIG.  i.— Original  time.  It  was  the  duty  of  the  labora- 
type  of  Plants  ^orv  assistants  to  set  up  the  battery 

accumulator.  . 

(About  i  full  °*  Bunsen  cells.     Huge  zincs  in  dilute 
size.)  sulphuric  acid  and  great  blocks  of  car- 

bon were  arranged  in  glass  jars  with  porous  cups, 
and  from  this  fuming  source  the  storage  cells  were 
charged  all  day,  to  be  used  the  day  following  in 
demonstrations  of  the  power  of  the  electric  current. 
After  the  charge  was  finished  the  big  Bunsens  were 
taken  apart  and  cleaned  up,  then  stored  away  until 


INTRODUCTORY  AND  HISTORICAL  3 

the  time  for  the  next  lecture  on  electric  currents 
approached. 

These  early  Plante  batteries  were  so  arranged  that 
they  could  be  easily  thrown  into  parallel  connection, 
and  in  this  way  they  could  be  charged  from  the 
Bunsen  battery  of  a  few  large  cells.  We  still  use 
one  of  these  batteries  of  20  cells,  dating  from  the 
early  eighties  or  earlier.  After  charge  was  com- 
plete the  simple  mechanism  permitted  all  the  cells 
of  the  set  to  be  connected  in  series  by  simply  turn- 
ing the  handle  through  90°,  and  clips  were  provided 
to  show  the  melting  of  wires  of  various  metals  by 
the  current. 

The  current  which  could  be  drawn  from  these 
small  sets  of  storage  cells  reached  its  maximum  at 
forty  or  fifty  amperes  —  an  enormous  value  then,  a 
mere  bagatelle  now,  for  we  have  electrolytic  cells 
and  electric  furnaces  which  require  tens  of  thou- 
sands of  amperes  for  their  operation.  Since  then 
lead  cells  have  grown  in  size  along  with  everything 
else  electrical,  and  I  have  seen  large  batteries  which 
can  furnish  thirty  or  forty  thousand  amperes  for  a 
short  time  and  ten  thousand  for  several  minutes  of 
discharge. 

2.  No  one  of  the  very  numerous  primary  cells 
which  have  been  devised  and  patented  has  ever 
reached  commercial  importance  for  the  heavier  work 
of  the  present  period,  though  a  few  have  survived  to 


4  STORAGE  BATTERIES 

do  the  lighter  tasks.  The  Leclanche  and  numerous 
similar  types  are  used  in  large  numbers  for  bell- 
ringing  installations  and  similar  open-circuit  appli- 
cations. And  the  dry  cell  has  a  very  large  and 
distinct  place  of  its  own  in  sparking  batteries  for 
motor  cars  and  boats  and  everywhere  that  internal 
combustion  engines  are  used.  Certainly  well  over 
ten  million  of  these  little  primary  cells  are  made  and 
used  each  year  in  the  United  States. 

From  the  beginning  of  the  nineteenth  century 
until  the  early  eighties  was  the  era  of  the  primary 
cell.  Then  came  the  dynamo  and  the  motor,  ac- 
companied by  improvements  in  our  main  prime 
source  of  power,  —  the  steam  engine,  —  and  the  stor- 
age cell  has  grown  along  with  all  of  these  in  a 
somewhat  subordinate  place.  It  is  a  mere  assistant, 
to  be  called  on  for  temporary  aid  in  time  of  need, 
either  to  help  over  an  ugly  peak  in  the  load  on  the 
prime  source,  or  as  insurance  to  be  called  in  when 
the  main  source  is  disabled  for  a  short  time,  and  its 
aid  is  often  quite  invaluable  under  these  conditions. 
As  a  real  factor  in  the  problem  of  prime  power 
sources  it  has  of  course  no  place  at  all. 

There  is  not  much  value  in  prophecies  about  scien- 
tific or  technical  things  and  no  particular  credit  is 
due  the  prophet  who  utters  them.  Nevertheless,  I 
feel  impelled  to  say  that  I  believe  the  day  of  the 
primary  cell  will  come  again.  From  every  funda- 


INTRODUCTORY  AND  HISTORICAL  5 

mental  and  theoretic  point  of  view  we  must  admit 
that  it  should  be  possible  to  make  a  primary  galvanic 
cell  which  should  be  more  efficient  than  a  steam 
engine  can  possibly  be  ;  more  flexible  as  a  primary 
source  of  power ;  a  better  appliance  in  every  way. 
3.  At  first  glance  a  lead-sulphuric  acid  storage 
cell  seems  a  very  simple  and  uninteresting  sort  of 
machine.  It  is  only  a  plate  of  lead  and  a  plate  cov- 
ered with  lead  peroxide,  dipping  into  rather  concen- 
trated sulphuric  acid.  But  for  those  who  make 
them  and  those  who  care  for  them  in  service  they 
become  much  more  complex  and  puzzling,  and  worth 
careful  consideration.  As  an  integral  and  essential 
part  of  many  power  arrangements  they  are  of  inter- 
est to  the  engineer  and  as  a  complex  of  puzzles  and 
problems  they  demand  attention  from  the  electro- 
chemist  and  the  physicist.  Many  books  have  been 
written  about  them,  some  purely  scientific  and  others 
nearly  purely  technical.  As  far  as  the  fundamental 
chemical  reaction  is  concerned  we  seem  to  be  on 
pretty  firm  ground,  and  there  is  every  reason  to  be- 
lieve that  we  know  how  the  cell  works.  But  there 
is  still  plenty  of  room  for  speculation  and  research 
on  the  more  minute  physical  changes  and  a  good 
many  questions  on  such  important  matters  as  forma- 
tion, cementing  of  pastes,  sulphation,  and  life  under 
various  conditions  cannot  even  now  be  answered 
very  clearly. 


6  STORAGE  BATTERIES 

A  very  large  number  of  combinations  have  been 
suggested  for  storage  battery  purposes  since  Plante 
began  to  study  his  cell  in  the  late  fifties,  but  until 
within  the  last  few  years  no  one  of  them  has  seemed 
able  to  meet  the  rather  difficult  and  peculiar  require- 
ments. Now  comes  the  Iron-Nickel  Oxide-Alkali 
combination  as  applied  by  Edison  in  this  country 
and  Jungner  on  the  continent  of  Europe,  and  this 
type  seems  destined  to  find  a  place  of  its  own  in 
light  traction  work.  But  by  far  the  greater  part  of 
all  storage  battery  plates  now  made  are  descendants 
of  the  original  Plante  type  —  hardly  recognizable 
with  their  highly  developed,  ribbed,  or  corrugated 
surfaces,  and  formed  in  the  factory  by  rapid  methods, 
but  still  "  Plante "  plates.  We  have  in  active  use 
in  our  own  laboratory  a  unique  battery  which  harks 
back  to  the  earliest  form.  It  has  twenty  thousand 
cells,  made  of  test  tubes,  and  the  plates  are  merely 
corrugated  strips  of  lead.  It  is  used  to  give  the 
small  currents  necessary  for  vacuum  tube  and  spark 
work,  and  it  was  formed  by  the  old  method  of  re- 
versals (see  page  179)  until  it  reached  the  needed 
capacity. 

4.  Faure  was  the  inventor  of  the  "  paste  "  plate, 
and  this  seemed  at  first  so  great  an  improvement 
that  prophets  were  not  wanting  to  predict  that  the 
older  type,  with  its  greater  weight,  comparatively 
small  capacity,  and  higher  cost,  would  be  completely 


INTRODUCTORY  AND  HISTORICAL  1 

ousted  by  the  new  invention.  These  prophecies 
have  not  been  fulfilled.  The.  paste  plate  has  been 
gradually  relegated  to  traction  work  and  to  duty 
where  weight  is  the  important  factor,  and  the  plates 
which  are  direct  descendants  of  the  Plante  originals 
do  the  really  hard  work.  It  took  much  experience 
and  expense  to  reach  the  decision  that  the  Faure 
plates  could  not  compete  in  the  more  strenuous  posi- 
tions, but  now  we  seem  to  appreciate  fairly  well  the 
limitations  of  both  types. 

5.  As  the  storage  battery  developed  to  a  point 
where  it  could  handle  real  power  loads,  there  came  a 
time  when  its  powers  were  somewhat  overestimated. 
It  was  suggested  for  many  positions  where  it  would 
have  been  quite  unfit  for  the  work  —  for  farm  pur- 
poses, for  motor  cycles,  and  even  for  airships.  For 
long-continued  discharge,  where  it  must  take  the 
place  of  the  prime  source  of  power  over  considerable 
periods  of  time,  the  storage  battery  is  often  a  cum- 
brous and  expensive  substitute  for  the  source  itself. 
But  for  many  kinds  of  work,  and  especially  where  a 
very  large  amount  of  power  is  needed  suddenly  or 
for  short  periods,  the  battery  is  the  ideal  machine. 
In  many  modern  plants  the  load  fluctuations  are  very 
great  —  a  thousand  per  cent  or  more,  and  this  within 
a  fraction  of  a  minute.  No  mechanical  arrangement 
can  absorb  this  and  regulate  the  load  on  the  power 
source  in  a  satisfactory  way.  But  a  storage  battery 


8  STORAGE  BATTERIES 

can,  for  there  is  hardly  a  limit  to  the  rate  at  which 
large-surface  Plante  plates  can  be  discharged  or 
charged  without  injury. 

In  certain  classes  of  work  —  in  submarines,  as  a 
source  of  under- water  power,  for  example  —  the  bat- 
tery is  an  absolute  necessity.  In  the  regulation  of 
irregular  loads  it  is  of  the  utmost  importance,  and  in 
emergency  or  "  stand-by  "  work  as  well.  Car  and 
train  lighting  systems  demand  its  use.  It  has  proven 
itself  economical  and  efficient  in  traction  work,  espe- 
cially for  electric  road  vehicles. 

Study  of  the  storage  battery  calls  for  attention  to 
two  rather  distinct  viewpoints — one  chemical,  the 
other  physical ;  and  these  will  be  found  of  "nearly 
equal  importance.  The  questions  about  the  funda- 
mental reactions,  and  many  others  as  well,  are  purely 
chemical.  Questions  about  the  life  of  the  cell,  and 
its  behavior  in  service,  are  nearly  purely  physical. 
In  manufacture  or  operation  the  chemical  side  must 
be  kept  in  mind,  but  the  anatomy  and  physiology 
(and  sometimes  the  pathology,  too)  of  the  individual 
plate  are  matters  of  prime  importance.  Underlying 
all,  we  will  need  as  a  foundation  for  study  the  funda- 
mental ideas  and  laws  of  general  electrochemistry. 

The  following  chapters  are  based  on  lectures  which 
have  been  given  for  the  last  few  years  at  Harvard 
University.  In  the  course  the  work  on  storage  cells 
is  preceded  by  study  of  the  general  theory  of  gal- 


INTRODUCTORY  AND  HISTORICAL  9 

vanic  cells,  and  the  simplest  of  this  theory  has  been 
included  in  this  book.  No  attempt  has  been  made 
to  give  any  of  the  detail  of  storage  battery  engineer- 
ing, but  only  to  introduce  the  reader  to  the  peculi- 
arities of  the  cell  itself. 


CHAPTER   II 
SOME  ELECTROCHEMICAL   FUNDAMENTALS 

6.  Theoretically  any  chemical  reaction  whatever 
which  takes  place  of  its  own  accord  can  be  so 
coupled  and  arranged  that  it  will  work  as  the  source 
of  energy  for  a  galvanic  cell.  Practically  there  are 
difficulties  which  exclude  a  large  percentage  of  the 
known  reactions  of  chemistry  from  such  service.  It 
is  also  true  that  a  great  many  of  the  combinations 
which  have  practical  value  as  primary  cells  can  be 
considered  theoretically  reversible  enough  to  be  used 
as  storage  cells.  As  a  matter  of  fact,  only  a  very 
few  of  the  cells  which  have  been  used  or  thought  of 
are  chemically  and  mechanically  reversible  enough 
to  fit  them  for  actual  use  as  storage  cells.  In  some 
cases  the  fault  is  in  the  reaction  itself,  and  the  cell  is 
not  chemically  reversible.  In  others,  the  reaction 
reverses  smoothly  enough,  but  the  materials  of  the 
cell  do  not  go  into  and  out  of  solution  well.  Here 
the  fault  is  a  mechanical  one.  As  far  as  the  general 
theory  is  concerned,  we  must  choose  fundamentals 
which  fit  all  the  cases,  even  those  which  cannot  be 
realized  practically. 

10 


SOME  ELECTROCHEMICAL  FUNDAMENTALS     11 

7.  Faraday's  Law.  —  We  have  one  general  funda- 
mental  electrochemical   law,   which   apparently   fits 
every  case,  and  which  brings  order  of  the  simplest 
kind  out  of  what  at  first  appeared  to  be  a  most  cha- 
otic mass  of  unrelated  material.     This  is  Faraday's 
law,  and  it  states  the  relation  between  the  quantity 
of  material  used  up  in  a  galvanic  cell  and  the  quan- 
tity of  electricity  which  can  be  obtained  from  it. 

This  law  says  :  — 

The  amount  of  each  substance  which  takes  part  in  an 
electrochemical  reaction  is  proportional  to  the  quantity 
of  electricity  which  passes  through  the  circuit. 

And  when  various  substances  enter  an  electrochemical 
reaction,  their  amounts  are  proportional  to  their  chemical 
equivalent  weights. 

Numerically,  and  in  terms  of  a  unit  later  to  be  de- 
fined :  — 

96,540  coulombs  pass  through  the  cell  and  the  external 
circuit  with  each  gram-equivalent  of  each  substance 
involved  in  the  reaction. 

8.  Faraday's   Definitions. — This  law  applies  to  elec- 
trolytes.    Faraday   himself   felt    the    necessity  of  a 
careful  set  of  definitions  for  the  new  ideas  involved 
in  this  law  and  its  application,  and  no  one  has  since 
given  better  ones,  so  we  shall  use  them  wherever  it 
is  possible  to  do  so. 

To  quote  Faraday  ("  Experimental  Researches," 
Series  VII,  1834):  — 


12  STORAGE  BATTERIES 

"...  In  place  of  the  term  pole,  I  propose  using 
that  of  Electrode,  and  I  mean  thereby  that  substance, 
or  rather  surface,  whether  of  air,  water,  metal,  or  any 
other  body,  which  bounds  the  extent  of  the  decom- 
posing matter  in  the  direction  of  the  electric  current. 
.  .  .  The  anode  is  therefore  that  surface  at  which 
the  electric  current,  according  to  our  present  ex- 
pression, enters.  It  ...  is  where  oxygen,  chlorine, 
acids,  etc.,  are  evolved;  and  is  against  or  opposite 
the  positive  electrode.  The  cathode  is  that  surface 
at  which  the  current  leaves  the  decomposing  body, 
and  is  its  positive  extremity  ;  the  combustible  bodies, 
metals,  alkalies,  and  bases,  are  evolved  there,  and  it 
is  in  contact  with  the  negative  electrode. 

"...  Many  bodies  are  decomposed  directly  by  the 
electric  current,  their  elements  being  set  free  ;  these 
I  propose  to  call  electrolytes.  .  .  . 

"  Finally,  I  require  a  term  to  express  those  bodies 
which  can  pass  to  the  electrodes.  ...  I  propose  to 
distinguish  such  bodies  by  calling  those  anions  which 
go  to  the  anode  of  the  decomposing  body,  and  those 
passing  to  the  cathode,  cations,  and  when  I  have 
occasion  to  speak  of  them  together,  I  shall  call  them 
ions.  Thus,  the  chloride  of  lead  is  an  electrolyte,  and 
when  electrolyzed  evolves  the  two  ions,  chlorine  and 
lead,  the  former  an  anion,  and  the  latter  a  cation.  ..." 

Figure  2  shows  the  different  parts  of  a  cell  as 
Faraday  denned  them. 


SOME  ELECTROCHEMICAL  FUNDAMENTALS     13 


These  definitions  of  Faraday's  were  made  with  the 
greatest  care,  but  since  they  were  formulated,  rather 
careless  use  has  sometimes  been  made  of  them.  Note 
the  term  anode.  It  is  the  surface  where  the  current 
enters  the  cell,  and  Faraday  meant  just  exactly  this 
whenever  he  used  the  word.  The  plates  of  a  cell  are 
not  anode  or  cathode  in  this  sense,  but  the  surface 
between  plate  and 

DIRECTION 

cell    solution     is.      OFCURRENT  ^ 

There  will  often  be 
occasion  to  retain 
this  strict  meaning 
of  the  word. 

Again,  an  electro- 
lyte is  the  body 
which  carries  the 
current  and  which 


EL 

TRODE            ELECTR 
INODE)          (CATHOD 

f 

E 

n 

I           ANIOM 

ELECTROLYTE 

CATHION 

— 

FIG.  2.  —  The  parts  of  an  electrolytic  cell. 


is  at  the  same  time  decomposed  by  it.  In  this  sense 
a  dry  salt  is  not  an  electrolyte,  but  a  solution  of  a 
metallic  salt,  or  a  molten  salt,  belongs  in  this  class. 

9.  Electrical  Units.  —  Before  we  can  apply  this  law 
of  Faraday's  we  should  review  a  few  more  electrical 
definitions.  In  what  is  called  the  practical  system, 
we  use  as  unit  of  quantity  of  electricity  one  coulomb. 
This  is  derived  from  the  unit  of  current,  the  ampere, 
and  one  coulomb  is  the  quantity  of  electricity  which 
passes  through  a  circuit  altogether,  when  a  current 
of  one  ampere  has  been  flowing  constantly  for  one 


14  STORAGE  BATTERIES 

second.  These  units  have  been  fixed  with  reference 
to  the  magnetic  effect  of  a  current  and  not  specially, 
with  reference  to  Faraday's  law.  It  is,  however,  an 
easy  calculation  to  state  them  in  terms  of  units  which 
bear  directly  on  electrochemical  effects.  Suppose  we 
have  in  the  circuit  an  amperemeter  which  measures 
the  current  in  amperes.  We  keep  the  current  con- 
stant and  note  the  entire  time  during  which  it  flows 
through  an  electrolytic  cell  in  which  silver  is  being 
deposited  from  silver  nitrate  solution.  We  will  find 
that  one  ampere  flowing  for  one  second  deposits 
0.00111775  gm.  of  silver.  The  equivalent  weight 
of  silver  is  in  this  case  the  same  number  of  grams  as 
its  atomic  weight,  and  has  the  value 

107.88  gm. 

The  number  of  coulombs  required  to  deposit  this 
weight  of  silver  is  then 

107.88 


0.0011175 


96,540  coulombs. 


This  same  number  of  coulombs  will  deposit  the 
equivalent  weight  of  any  other  metal  which  can  be 
electroplated  in  the  same  way,  and  it  is  the  electro- 
chemist's  unit  of  quantity  of  electricity. 

If  the  silver  were  to  be  used  in  a  galvanic  cell  as  a 
source  of  power,  exactly  the  same  relation  holds  be- 
tween the  weight  of  silver  and  the  quantity  of 
electricity — 107.88  gm.  of  silver  always  travels 


SOME  ELECTROCHEMICAL    FUNDAMENTALS      15 

through  an  electrolyte  and  dissolves  or  precipitates 
at  the  electrode  in  company  with  96,540  coulombs. 

Silver  ion  is  univalent,  and  the  equivalent  weight 
is  the  same  as  the  atomic  weight.  In  most  of  its 
reactions,  chemical  and  electrochemical,  copper 
forms  a  bivalent  ion.  This  means, that  in  company 


'A  B 

FIG.  3.  —  Diagram  of  apparatus  to  show  Faraday's  law. 

with  the  atomic  weight  of  copper  (63.6  gm.) 
twice  96,540  coulombs  pass  through  the  circuit; 
so  the  equivalent  weight  of  copper  is  31.8  gm., 
and  this  is  the  electrochemist's  unit  weight  of  copper. 
10.  Experimental  Arrangement  for  Faraday's  Law.  — 
Figure  3  gives  diagrammatic  representation  of  an 
experiment  to  illustrate  Faraday's  law.  Current 
is  supplied  by  the  battery  A  and  passes  first  through 
the  tangent  galvanometer  B,  which  measures  it,  and 
then  on  through  the  various  cells  in  which  electro- 


16  STORAGE  BATTERIES 

chemical  reactions  take  place.  In  (7,  a  molten  salt, 
silver  chloride,  for  example,  is  decomposed.  D 
might  represent  a  copper  coulometer,  in  which  copper 
is  dissolved  at  one  electrode  and  precipitated  at  the 
other.  The  same  arrangement  might  be  used  for 
many  other  metals.  HI  is  one  form  of  silver 
coulometer,  and  here  the  current  enters  at  a  silver 
anode,  which  goes  into  solution,  and  leaves  the  cell 
at  the  surface  of  a  platinum  crucible  (cathode)  on 
which  silver  is  deposited.  The  electrolyte  is  a 
strong  solution  of  silver  nitrate.  Last  in  the  row 
is  a  gas  coulometer  jP,  containing  dilute  acid  or 
alkali  as  electrolyte  and  having  platinum  electrodes. 
Oxygen  gas  is  formed  at  the  anode,  the  electrode 
where  the  current  enters  the  apparatus,  and  hydro- 
gen gas  is  evolved  at  the  other  electrode. 

Suppose  we  have  sent  a  constant  current  of  one 
ampere  through  the  circuit  for  96,540  sec.  We  have 
weighed  the  electrodes  before  and  after  the  passage 
of  this  current,  and  we  have  measured  the  volumes  of 
the  two  gases  produced.  We  should  find :  — 

1.  At  (7,  107.88  gm.  of  silver  dissolved  from  the 
wire  at  which  the  current  enters  the  cell   and  the 
same  weight  of  silver  deposited  on  the  other  wire. 
The  electrolyte  remains  unchanged. 

2.  At  Z>,  31.8  gm.  of  copper  dissolved  at  one  plate 
and  precipitated  at  the  other.     No  change   in   the 
electrolyte. 


SOME  ELECTROCHEMICAL   FUNDAMENTALS      17 

3.  At  E,  the  same  amounts  of  silver  dissolved  and 
precipitated  as  in  C. 

4.  At   F,  8   gm.  of   oxygen   formed,  or  5.6  1.  if 
measured  at  0°  C.  and  760  mm.  pressure,  and  at  the 
other  electrode,  1  gm.  of  hydrogen,  having  a  volume 
of  11.2  1. 

5.  Inside   the   cells   at  A,  there  will   have   been 
exactly  equivalent  effects,  and  they  will  be  the  same 
in  each  cell.     Whatever  the  materials  of  the  anode 
and  cathode,  equivalent  weights  of  each  will  have 
entered  into  reaction,  for  as  far  as  the  application 
of  Faraday's  law  is  concerned,  it  makes  no  difference 
whether  work  is  performed  as  the  result  of  a  reac- 
tion, or  must  be  performed  from  without  in  order  to 
make  the  reaction  take  place.     The   law  describes 
every  electrochemical  reaction,  and  has  been  shown 
to  be  as  exact  as  any  law  we  have. 

11.  Practical  Application.  —  Let  us  examine  some 
applications  of  this  law.  A  great  deal  of  copper  is 
purified  in  this  country  by  an  electrolytic  process. 
It  is  interesting  to  calculate  the  quantity  of  electric- 
ity needed  to  deposit  a  pound  of  copper  in  this  way. 

1  Ib.  =  453  gm. 

96,540  coulombs  deposit  31.8  gm. 
We  therefore  need 
453 


31.8 


X  96,540  =  1,376,000  coulombs  per  pound. 


18  STORAGE  BATTERIES 

Since  an  ampere  is  1  coulomb  per  second,  it  will 
require 

1,376,000      ouo 

—  =  382  ampere-hours 
ooOO 

to  deposit  a  pound  of  copper  in  a  single  cell.  382 
amperes  deposit  1  Ib.  of  copper  per  hour  in  a  single 
cell,  and  if  we  wish  to  obtain  a  ton  of  copper  per 
hour  in  such  a  cell,  it  would  take  a  current  of  nearly 
760,000  amperes  to  give  the  desired  result.  As  a 
matter  of  fact  cells  of  this  size  are  never  used.  It 
is  better  to  arrange  a  number  of  cells  in  series,  so 
that  the  current  flows  through  one  after  the  other 
and  produces  the  same  effect  in  each.  The  yield  of 
copper  is  then  to  be  found  by  multiplying  the  yield 
per  cell  by  the  number  of  cells. 

The  atomic  weight  of  lead  is  about  207,  and 
it  is  formed  from  a  bivalent  ion,  so  the  equiva- 
lent weight  of  lead  is  103.5.  Rather  more  than 
three  times  as  much  lead  as  copper  is  deposited  by 
the  same  quantity  of  electricity.  The  calculation  is 

x  96,540  =  422,000  coulombs  per  pound  of  lead. 


12.  Electrolysis  in  the  Daniell  Cell.  —  In  the  Daniell 
type  of  primary  cell  the  chemical  reaction  is  a  very 
simple  one  :  Copper  is  deposited  as  metal  from  cop- 
per sulphate  solution  ;  zinc  (metal)  passes  into  solu- 
tion as  zinc  sulphate. 

Zn  +  CuSO4  =  ZnSO4  +  Cu. 


SOME  ELECTEOCHEMICAL  FUNDAMENTALS      19 

The  reaction  is  indicated  in  the  diagram  of  Fig- 
ure 4. 

How  many  ampere-hours  can  we  get  from  a  Daniell 
cell  per  pound  of  zinc? 

The  atomic  weight  of  zinc  is  65.4,  and  it  acts  as  a 
bivalent  ion,  so  we  will  get  96,540  coulombs  from 


ZINC 

1 

PART 

ITION       c 

ra 

op 

PER 

ZINC 

su 

LPHATE 

COPPER 

s 

ULPHATE 

SO 

LU 

riON 

SOI 

in 

P 

ION 

65.4 


FIG.  4.  —  Diagram  of  the  reaction  in  a  Daniell  cell. 


=  32.7  gm.  of  the  metal.     A  pound  is  453  gin. 


Per  pound  of  zinc  we  can  therefore  obtain 
453 


32.7 


x  96,540  =  1,337,000  coulombs, 


and  since   an   ampere-hour   is    3600    coulombs,    one 
pound  of  zinc  will  give  372  ampere-hours. 

We  can  get  this  same  number  of  ampere-hours  per 
pound  of  zinc  in  any  galvanic  arrangement  whatever, 
and  it  requires  the  same  number  to  deposit  a  pound 
of  zinc  electrolytically  from  its  solution. 


20  STORAGE  BATTERIES 

How  much  copper  sulphate  must  we  supply  dur- 
ing this  time  to  keep  the  copper  side  of  the  Daniell 
cell  active  ? 

Its  formula  is  CuSO4  +  5  H2O,  and  the  total  weight 
equivalent  to  65.4  gm.  of  zinc  is  therefore  249  gm. 
Copper  ion  passes  through  a  bivalent  step  in  its 
deposition  as  metallic  copper,  so  it  requires  -|^  = 
124.5  gm.  of  "blue  vitriol"  to  give  96,540  coulombs. 
To  furnish  1,337,000  coulombs  we  must  use 

x  124.5  =  1725  gm.,  or  3.8  Ib. 

Since  all  our  electrochemical  reactions  are  really 
only  chemical  ones  arranged  in  such  a  way  that  they 
furnish  or  require  a  current  of  electricity,  we  could 
calculate  the  amount  of  copper  sulphate  needed  for 
our  run  with  the  Daniell  cell  directly  from  the  pre- 
ceding figure  for  the  deposition  of  metallic  copper  in 
the  purification  process. 

/>o     (* 

A  pound  of  blue  vitriol  contains  —  —  =  0.255  Ib. 


of  copper,  and  we  found  that  it  required  382  ampere- 
hours  to  deposit  a  pound  of  copper.  The  same 
quantity  of  electricity  will  pass  through  the  Daniell 
cell  with  a  pound  of  copper,  and  to  get  1,337,000 
coulombs  from  the  cell  we  must  deposit 

1*887.000  =  Q.972  Ib.  of  copper. 
382  x  3600 


SOME  ELECTROCHEMICAL  FUNDAMENTALS      21 

This  amount  of  copper  is  contained  in  3.8  Ib.  of  blue 
vitriol. 

13.  Electrochemical  Units.  —  It  is  evident  that  the 
96,540  coulomb  unit  which  the  electrochemist  is 
obliged  to  use  is  a  rather  cumbrous  one  and  leads  to 
large  numbers.  If  we  had  the  choosing  of  our  own 
unit  we  would  of  course  make  96,540  coulombs  =  1 
electrochemical  unit  of  quantity  of  electricity,  and 
then  the  calculation  for  copper  would  look  like 
this  :  — 

63. 6  g.  Cu~2  units, 
1  Ib.  copper  ~  14. 24  units, 

and  for  zinc  it  would  be  equally  simple.  But  elec- 
trochemistry is  not  a  big  anough  branch  of  science 
to  be  able  to  dictate  units  to  the  dynamos  which  fur- 
nish the  current,  and  we  must  be  content  to  accept 
the  electrical  engineer's  unit. 

In  every  case  it  is  necessary  to  know  the  complete 
and  exact  chemical  reaction  with  which  we  are  deal- 
ing before  we  can  apply  our  law,  for  it  very  often 
happens  that  metals  carry  different  multiples  of  the 
unit  quantity  of  electricity  with  them  in  different 
chemical  reactions,  and  they  sometimes  complicate 
things  still  further  by  changing  the  number  of  units 
carried  as  the  concentration  of  the  solution  from 
which  they  are  deposited  is  changed.  But  if  we 
arrange  to  have  the  conditions  in  the  cell  constant 


22  STORAGE  BATTERIES 

and  have  once  found  the  correct  chemical  reaction, 
the  law  can  always  be  applied  without  fear  of  error. 

14,  Electromotive  Force.  —  Faraday's  law  gives  a 
complete  statement  of  the  quantity  of  electricity 
which  accompanies  the  reaction  of  gram-equivalent 
weights  of  various  substances  in  any  galvanic  com- 
bination or  electrolytic  cell.  But  it  can  tell  us  no 
more  than  this.  It  says  nothing  about  the  amount 
of  work  we  can  do  with  this  amount  of  electricity, 
nor  about  the  amount  of  work  we  must  do  to  cause 
the  separation  of  a  gram-equivalent  of  a  metal  from 
solution.  The  driving  force  of  the  chemical  reaction 
and  the  corresponding  electromotive  force  of  the  cell 
are  specific  for  each  reaction  and  cannot  be  calcu- 
lated by  any  inclusive  general  law.  The  driving 
force  is  called  the  chemical  potential  of  the  reaction, 
and  it  can  be  very  conveniently  and  accurately 
measured  by  coupling  the  reaction  into  the  form  of  a 
galvanic  cell  and  measuring  the  electromotive  force. 

Very  early  in  the  development  of  galvanic  elec- 
tricity Volta  found  that  the  various  metals  could  be 
arranged  in  a  series,  such  that  the  most  favorable 
combinations  for  producing  current  were  to  be  made 
by  choosing  metals  as  far  apart  as  possible  in  the 
series.  Better  results  were  obtained  from  cells  using 
zinc  and  copper  than  from  those  using  iron  and  copper, 
or  zinc  and  tin.  We  know  now  that  not  only  the 
metal,  but  the  whole  reaction  must  be  taken  into  ac- 


SOME  ELECTROCHEMICAL  FUNDAMENTALS     23 

count,  but  the  "  Voltaic  series  of  the  metals,"  as  it  is 
called,  gives  an  approximate  view  of  the  matter. 

It  was  found  very  early  that  more  work  could  be 
obtained  from  a  pound  of  zinc  in  a  cell  where  copper 
is  deposited  at  the  cathode,  than  from  a  cell  where 
iron  is  used  in  the  same  way.  The  same  quantity  of 
zinc  is  used  up  in  each  case,  and  since  we  get  different 
results  in  the  various  combinations,  there  must  be 
some  other  factor  of  importance  and  some  other  law 
besides  Faraday's  to  be  considered. 

Suppose  we  have  a  very  large  Daniell  cell,  where 
the  reaction 

Zn  +  CuSO4  =  Cu  +  ZnSO4 

is  taking  place.  We  choose  a  big  cell  in  order  that 
we  may  send  96,540  coulombs  through  it  without  any 
danger  of  changing  the  concentrations  in  the  differ- 
ent parts  of  the  cell.  When  this  quantity  of  elec- 
tricity has  passed  through  the  cell,  32.7  gm.  of  zinc 
have  gone  into  solution  at  the  anode  and  have  become 
zinc  ion.  During  this  same  time  31.8  gin.  of  cop- 
per ion  have  changed  into  metallic  copper.  The 
SO4  part  of  the  reaction  has  not  been  affected  at  all. 
Electrochemically  we  could  write  the  reaction 


15.  Ions.  —  The  small  sign  +  indicates  that  the  sub- 
stance carrying  them  is  an  ion  and  that  it  moves 
toward  the  cathode  —  it  is  a  cation.  Two  of  them 


24  STORAGE  BATTERIES 

indicate  that  this  particular  ion  carries  with  it  per 
gram-atom  twice  the  unit  quantity  of  electricity 
(2  x  96,540  coulombs).  The  SO4  ion  (SO4— ),  which 
remains  unchanged  in  this  particular  case,  carries 
two  times  the  unit  quantity  also,  but  toward  the 
anode.  It  is  an  anion.  And  in  chemical  parlance 
both  of  these  are  divalent  ions. 

Now  suppose  we  connect  the  cell  with  an  external 
source  of  current  and  send  96,540  coulombs  through 
it  in  the  opposite  direction.  32.7  gm.  of  zinc  will 
deposit  on  the  zinc  plate,  —  now  the  cathode,  —  and 
31.8  gm.  of  copper  will  go  into  solution  at  the  copper 
plate,  —  now  the  anode.  By  the  time  we  have  sent 
our  unit  quantity  through  the  cell  it  has  been  com- 
pletely restored  to  its  original  condition.  The  case 
of  the  Daniell  cell  is  theoretical  rather  than  practical, 
for  zinc  does  not  behave  very  well  when  it  is  forced 
out  of  solution.  It  grows  in  sponge  and  trees  and 
often  reaches  across  to  the  other  plate  and  short- 
circuits  the  cell.  But  we  have  chosen  our  cell  so 
large  that  this  does  not  bother  us,  and  the  Daniell 
cell  can  be  considered  completely  reversible  in  its  re- 
actions. It  might  therefore  be  used  as  an  accumu- 
lator. 

16.  Other  Electrical  Units.  —  Besides  the  coulomb, 
we  have  been  supplied  with  two  other  units,  and  these 
fortunately  fit  electrochemical  needs  pretty  well  with- 
out requiring  so  many  figures.  One  of  these  is  the 


SOME  ELECTROCHEMICAL  FUNDAMENTALS     25 

volt,  the  unit  of  difference  of  potential,  and  the  other 
is  the  ohm,  the  unit  of  resistance. 

The  following  terms  and  relations  are  important: — 

Coulomb  (<?)  Quantity  of  electricity. 

Ampere  (i)  Current. 

Volt  (0)  Difference  of  potential. 

Joule  (/)  Energy. 

1  volt-coulomb  =  1  joule. 

watt  =  rate  of  furnishing  energy. 
1  volt-ampere  =  1  watt. 
1  joule  per  second  =  1  watt. 

3600  coulombs  =  1  ampere-hour. 
1000  watts  =  1  kilowatt,  KW. 
746  watts  =  1  horse  power,  H.P. 
746  x  3600  joules  =  1  horse-power  hour,  H.P.H. 

Beside  our  units  we  can  also  get  instruments  for 
measuring  them  from  the  electromagnetic  branch  of 
electrical  science.  If  we  borrow  a  voltmeter  from 
our  neighbor,  the  electrical  engineer,  and  apply  its 
terminals  to  our  Daniell  cell,  we  measure  what  is 
called  its  electromotive  force  in  volts.  The  volt- 
meter reads  about  1.1  volts. 

17.  Electrical  Energy.  —  We  can  now  calculate  the 
electrical  energy  obtainable  from  this  cell.  By  ex- 
pending 32.7  gm.  of  zinc  and  31.8  gm.  of  copper  ion 
we  can  expect  to  get  1.1  X  96,540  volt-coulombs 
(joules)  with  which  to  do  useful  work  outside  the 


26  STOBAGE  BATTERIES 

cell.  If  we  are  sending  current  through  the  cell  in 
the  opposite  direction,  we  can  reverse  the  reaction 
and  return  the  cell  to  its  original  condition  by  an 
expenditure  of  the  same  amount  of  work. 

We  can  now  calculate  both  work  and  power. 
How  many  horse-power  hours  can  be  obtained  from 
a  Daniell  cell  per  pound  of  zinc  ? 

32.7  gm.  of  zinc  give 

1.1  x  96,540  =  106,300  joules. 
1  H.P.H.  is 

746  x  3600  =  2,683,000  joules. 
1  Ib.  of  zinc  will  give 

^!  x  106,300  =  1,472,000  joules. 
oZ.t 

1  Ib.  of  zinc  will  therefore  give 

1,472,000  =  Q55HRH 

2,683,000 

Or,  we  must  use  a  little  less  than  2  Ib.  of  zinc  per 
horse-power  hour. 

Other  forms  of  zinc-consuming  cells  were  formerly 
much  in  use,  and  some  of  these  had  electromotive 
forces  as  high  as  2  volts.  One  of  these  would  re- 
quire only 

—  Ib.  of  zinc  to  produce  0.55  H.P.H., 

a 

and  we  would  need  only  1.07  Ib.  of  zinc  per  horse- 
power hour  in  the  case  of  one  of  these  cells. 


SOME  ELECTROCHEMICAL   FUNDAMENTALS      27 

Resistance.  —  The  unit  of  resistance  has  an  inter- 
esting and  simple  relation  to  the  units  of  current 
and  voltage.  What  is  called  Ohm's  law  states 

electromotive  force  in  volts 
current  in  amperes  =  -        — ; —      — - — =—        — 

resistance  in  ohms 

Or,  an  electromotive  force  of  1  volt  will  send  a  cur- 
rent of  1  ampere  through  a  circuit  having  a  resist- 
ance of  1  ohm. 

A  column  of  mercury  106.3  cm.  long  and  one 
square  millimeter  in  cross-section  has  a  resistance 
of  1  ohm.  A  good-sized  copper  wire  has  a  resistance 
of  an  ohm  for  a  length  of  a  thousand  feet  or  so. 

18.  If  it  is  desired  to  furnish  0.5  H.P.  from  a 
single  Daniell  cell,  at  what  rate  must  zinc  dissolve  ? 

0.5  H.P.  is  373  watts  (volt-amperes)  (volt-cou- 
lombs per  second).  Our  cell  gives  1.1  volts  and 
must  therefore  give  a  current  of  349  amperes  (349 
coulombs  per  second). 

32.7  gm.  of  zinc  furnish  96,540  coulombs. 

349 

We   must   therefore   furnish  - — — —  x  32.7    gm.    of 

96,540 

zinc  per  second;  0.118  gm.  of  zinc  per  second  or 
425  gm.  per  hour  will  give  0.5  H.P. 

If  we  set  up  a  whole  row  of  Daniell  cells  as  a 
battery,  and  draw  our  0.5  H.P.  from  this,  we  will  be 
much  nearer  the  practical  truth,  for  it  would  take 
an  enormous  cell  to  give  350  amperes,  owing  to  the 


28  STORAGE  BATTERIES 

rather  high  internal  resistance  caused  by  the  porous 
cup. 

19.   Cells  in  Series  and  Parallel.  —  Suppose  we  have 
100  cells  in  our  battery,  each  with  an  electromotive 


FIG.  5.  —  Cells  connected  in  parallel.     The  effect  is  the  same  as  though 
all  the  plates  were  placed  in  one  large  cell. 

force  of  1.1  volts.     If  they  are  connected  so  that  the 
zinc  of  each  cell  is  fastened  to  the  copper  of  the  next 


FIG.  6.  —  Cells  connected  in  series. 

one  as  shown  in  Figure  6,  their  electromotive  forces 
will  add,  and  our  whole  battery  will  have  an  electro- 
motive force  of  110  volts.  To  get  373  watts  or  0.5 
H.P.  we  need  to  draw  only  -f^o  =  ^«4  amperes  from 


SOME  ELECTROCHEMICAL  FUNDAMENTALS      29 

our  battery,  and  this  would  not  be  an  unreasonable 
current  for  large  Daniell  cells.  You  will  notice  that 
the  total  weight  of  zinc  dissolved  and  copper  de- 
posited is  exactly  the  same  as  though  it  had  taken 
place  in  one  huge  cell,  though  now  it  is  distributed 
over  100  cells. 

In  our  very  first  problem,  on  page  17,  where  we 
calculated  the  current  required  to  deposit  a  ton  of 
copper  per  hour  by  electrolysis,  we  obtained  a  value 
for  a  single  huge  cell.  Practically  copper  would 
never  be  purified  in  that  way,  for  the  voltage  nec- 
essary to  deposit  copper  is  not  more  than  0.3  volt, 
and  it  is  not  feasible  to  build  a  generator  to  work  at 
that  voltage.  Besides,  it  is  not  necessary,  as  it  is  just 
as  well  to  work  a  number  of  electrolytic  cells  in 
series  like  a  battery.  In  many  copper  refineries  200 
such  cells  are  used  and  a  current  of  perhaps  4000 
amperes  is  sent  through  the  whole  series.  This  re- 
quires a  generator  capable  of  giving  this  number  of 
amperes  at  about  60  volts,  and  the  power  required  is 
therefore  240  KW.  The  copper  deposited  has  the 
same  weight  as  though  800,000  amperes  were  sent 
through  a  single  cell,  and  is  therefore  a  little  over  a 
ton  per  hour. 


CHAPTER  III 

ABOUT  IONS 

20.  All  electrochemical  processes  follow  Faraday's 
law  absolutely  as  far  as  any  one  can  find  out,  and 
they  therefore  invariably  depend  on  ions  in  the  sim- 
ple sense  in  which  Faraday  himself  used  this  term 
(page  12).      There  is,  nowadays,  a  whole   field   of 
science  which  has  to  do  with  the  study  of  the  ions 
of  gases,  and  some  of  the  most  interesting  and  sug- 
gestive of  all  modern  developments  are  being  made 
in  this  field.     These  hypotheses  and  theories,  now 
just  being  cleared  of   their  mysteries  and  made  a 
part  of  general  science,  will  no  doubt  some  day  be- 
come a  safe  and  useful  basis  for  the  study  of  electro- 
chemistry.    But  for  the  present,  at  least,  we  will  be 
safer  if  we  stick  close  to  Faraday,  and  call  our  ions 
"...  those  bodies  which  can  pass  to  the  electrodes." 
We  shall  meet  with  rather  strange  ones  when  we 
come  to  the  lead  storage  cell  itself,  and  some  general 
knowledge  of  the  simpler  sorts  will  be  found  a  useful 
introduction. 

21.  Conductance  by  Ions.  —  In  the  first  place,  the 
ions  are  already  there  in  a  solution  of  a  metallic  salt 

30 


ABOUT  IONS  31 

or  in  a  molten  electrolyte.  They  are  not  produced 
by  the  action  of  the  current.  And  they  are  able  to 
begin  carrying  electricity  toward  the  electrodes  as 
soon  as  the  circuit  is  closed.  It  is  also  certain  that 
they  do  all  the  work  of  carrying  the  current  through 
the  cell.  These  last  two  statements  are  merely  an- 
other way  of  stating  the  extreme  accuracy  of  Fara- 
day's law.  No  current  seems  to  pass  through  an 
electrolyte  unaccompanied  by  the  movement  of  an 
exactly  equivalent  amount  of  each  of  two  ions  — 
an  anion  and  a  cation. 

Water  itself  is  a  conductor  of  the  electrolytic  kind. 
It  has  a  high  resistance,  to  be  sure,  but  it  does  con- 
tain small  concentration  of  the  ions  H+  and  OH~. 
It  is  chiefly  remarkable  for  the  aid  it  gives  to  other 
substances  in  the  process  of  ionization.  Metallic 
salts,  and  acids  and  bases  as  well,  are  famous  carriers 
of  current  when  they  are  in  solution  in  water,  and 
they  always  follow  Faraday's  law.  Many  of  them 
are  also  good  conductors  in  the  molten  state,  and 
their  ions  pass  to  the  electrodes  under  these  circum- 
stances just  as  well  as  they  do  in  water. 

22.  Chemical  Facts  connected  with  Ions.  —  Since 
Faraday  offered  his  suggestion  about  the  names  to 
be  used  in  describing  the  process  of  electrolysis,  and 
gave  to  the  ions  their  simple  definition,  much  of 
chemistry  has  been  restated.  The  general  facts 
about  solutions,  and  especially  those  which  have  to 


32  STORAGE  BATTERIES 

do  with  ions,  even  apart  from  their  power  of  carry- 
ing a  current,  have  been  brought  together  into  one 
of  the  most  united  and  easy  branches  of  the  science 
of  chemistry.  Let  us  consider  a  few  of  the  simpler 
generalizations.  All  acids  in  water  solution  contain 
hydrogen  ion,  H+,  and  their  acid  properties  are  de- 
pendent on  its  presence  and  are  measured  by  its 
concentration.  All  bases  in  solution  contain  OH~ 
(hydroxyl  ion).  Solutions  of  metallic  salts  usually 
contain  an  ion  produced  from  the  metal,  like  Cu++, 
Zn++,  Ag+,  K+,  Al+++,  Pb++,  and  an  ion  formed  from 
the  other  part  of  the  salt  — Cl",  Br~,  NO3~,  C1O4~, 
SO4  ,  CrO4  .  We  quickly  get  into  the  habit  of 
thinking  about  the  particular  ion  we  want  for  any 
special  set  of  properties  it  may  have,  and  I  have 
often  heard  a  student  just  beginning  chemistry  — 
one  who  had  not  the  slightest  idea  of  Faraday's  law 
or  of  any  electrochemical  theory  —  say  to  his  neigh- 
bor, "  Pass  the  copper  bottle,"  when  he  meant  copper 
sulphate  or  nitrate  or  any  other  soluble  copper  salt. 
He  needed  copper  ion  for  his  experiment,  and  in 
the  same  way  a  more  advanced  student  will  ask, 
"Have  you  some  acid?"  when  he  wants  hydrogen 
ion.  In  neither  of  these  cases  does  the  other  ion, 
which  is  sure  to  be  present,  interest  the  chemist,  pro- 
vided it  has  not  some  special  peculiarity  of  its  own. 
But  if  the  other  ion  can  form  a  difficultly  soluble  salt 
with  one  of  those  in  his  test  tube,  he  will  be  more  ex- 


ABOUT  IONS  33 

plicit  in  stating  the  kind  of  copper  salt  solution  or  the 
kind  of  acid  solution  he  needs.  If  you  will  think  over 
your  own  experiences  with  solutions  of  acids,  bases, 
and  metallic  salts,  you  will  see  that  the  chemistry  of 
aqueous  solutions  can  all  be  brought  into  the  easiest 
form  by  a  classification  of  the  properties  of  ions. 
Besides  this,  one  only  needs  knowledge  of  the  solu- 
bilities of  salts  to  have  a  pretty  full  command  of  the 
facts  about  aqueous  solutions. 

This  same  statement  is  almost  equally  true  of 
electrochemistry.  A  current  only  passes  through 
a  solution  when  two  ions  carry  it.  These  ions  pass 
back  and  forth  at  the  electrodes  and  send  their  quota 
of  electricity  out  through  the  wires  of  the  circuit  as 
a  current.  Each  ion  travels  through  the  electrolyte 
with  its  own  special  velocity  and  carries  a  fraction 
of  all  the  current  flowing  which  is  proportional  to 
this  velocity.  If  we  had  space  for  a  really  complete 
theory  of  galvanic  cells,  we  would  need  careful  study 
of  the  changes  which  take  place  at  various  parts  of 
such  a  cell  as  the  result  of  differences  in  ionic  migra- 
tion velocity.  We  should  at  the  same  time  find 
some  very  simple  and  interesting  generalizations 
about  the  part  played  by  the  individual  ions  in  elec- 
trolytic conductivity. 

23.  The  Ionic  Theory.  —  In  some  of  our  explanations 
we  shall  feel  the  need  of  a  much  more  minute  and 
detailed  picture  of  what  happens  than  can  be  ob- 


34  STORAGE  BATTERIES 

tained  by  adhering  closely  to  Faraday's  careful  defi- 
nition of  an  ion.  We  shall  need  to  bring  in  occa- 
sionally a  more  hypothetical,  or  rather  theoretical, 
ion  than  Faraday's.  This  does  no  harm,  for  more 
and  more  proof  of  the  general  usefulness  and  truth 
of  the  general  theory  of  ions  is  being  accumulated 
every  day.  The  step  from  Faraday  to  the  theoretical 
picture  is  not  a  great  one. 

Ions  are,  in  this  picture,  parts  of  molecules,  each 
one  connected  with  a  definite  and  constant  quantity 
of  electricity,  either  positive  or  negative.  If  we 
collect  enough  of  these  little  carriers  to  make  a 
gram-equivalent,  arid  send  them  along  to  discharge 
against  an  electrode,  96,540  coulombs  will  pass  this 
surface  and  flow  out  through  the  wires  of  the  exter- 
nal circuit.  At  the  same  time  enough  of  the  ions  of 
opposite  sign  to  carry  the  same  quantity  of  electric- 
ity will  have  been  discharged  at  the  other  electrode. 
Faraday's  ion  was  singular,  and  we  shall  refer  to  an 
ion  as  it  when  we  need  no  further  statement  than 
that  involved  in  Faraday's  law.  When  we  want  to 
describe  the  more  complicated  changes  about  the 
electrodes,  we  shall  make  use  of  the  other  picture  and 
refer  to  the  ions  of  copper  or  silver,  using  the  plural 
and  picturing  an  electrolyte  filled  with  them,  each 
carrying  its  unit  quantity  of  electricity,  and  all 
swarming  toward  the  electrodes  when  current  passes. 

The  electrolyte  which  is  used  in  a  storage  cell  is 


ABOUT  IONS  35 

a  rather  concentrated  solution  of  sulphuric  acid  in 
water.  It  contains  considerable  concentrations  of  the 
ions  H+  and  SO4  ,  and  these  do  the  carrying  of  the 
current  across  the  space  between  the  electrodes. 
During  the  passage  of  current  in  either  direction,  H+, 
the  cation,  moves  toward  the  cathode,  whichever 
plate  this  may  happen  to  be,  and  at  the  same  time 
SO4  ,  the  anion,  moves  toward  the  anode.  The 
direction  of  flow  of  the  current  is  reversed  when  the 
cell  passes  from  charge  to  discharge  and  the  direction 
of  the  motion  of  the  ions  changes  also. 

24.  Migration  Velocities.  —  If  both  the  ions  moved 
through  the  electrolyte  with  the  same  velocity,  there 
would  never  be  any  difference  in  ionic  concentrations 
in  any  part  of  the  cell.  It  was  found  a  long  time  ago 
that  considerable  differences  are  set  up  during  elec- 
trolysis, and  from  measurements  of  these  concentration 
differences  it  was  found  possible  to  calculate  the 
relative  migration  velocities  of  all  the  ions.  Later 
the  actual  velocity  with  which  an  ion  passed  through 
the  solution  was  measured,  and,  of  course,  as  soon 
as  the  real  velocity  of  motion  of  one  single  ion  was 
found,  all  the  other  velocities  could  be  calculated  from 
the  relative  numbers  found  by  means  of  the  concen- 
tration differences. 

H+  ion  moves  through  the  solution  about  five  times 
as  fast  as  SO4~~.  Figures  7  and  8  show  the  condition 
of  things  in  the  cell  (7)  before  any  current  has 


36  STORAGE  BATTERIES 

passed,  and  (8)  after  6  SO4  ions  have  separated  at 
the  anode. 

We  must  remember  that  the  number  of  +  and  — 
ions  must  always  be  the  same  at  any  point  in  the  cell. 

ooooooooiooooojoooooooo 

I  1 

OOOOOOOOOOOOOOOOlOOOOOOOOOODOOOOOOOOOOOOOOO 

I  I 

FIG.  7.  —  Diagram  of  ion  concentrations  in  an  electrolytic  before  cur- 
rent begins  to  flow. 

The  attraction  of  the  -f  and  —  charges  on  these  very 
small  bodies  is  so  great  that  we  can  never  hope  to  get 
more  than  the  most  minute  concentration  of  any  one 
kind  of  ion  off  by  itself,  and  we  have  very  good  evi- 
dence that  our  solutions  are  everywhere  electrically 


oo 
oo 
oo 


o  o  o  ioooooiooooooo 

000000   1000000000000000000000000 

I  I 


FIG.  8.  —  The  cell  of  Figure  7  after  six  SOr  ~  ions  have  left  the  electro- 
lyte at  the  anode. 

neutral,  which  means  that  the  concentration  of  +  and 
—  ions  is  everywhere  the  same. 

This  statement  suggests  the  question  :  How  can  a 
slow-moving  ion  get  to  its  electrode  fast  enough  to 
keep  up  the  supply  there  ? 

And  the  answer  is  that  it  cannot  keep  the  concen- 
tration at  its  original  value.  During  electrolysis  the 


ABOUT  IONS  37 

electrolyte  about  the  place  where  the  slow-moving 
ion  is  going  out  of  solution  is  depleted.  Its  concen- 
tration becomes  less  and  less,  until  diffusion  finally 
stops  the  dilution.  In  the  meantime  the  fast-moving 
ion  has  become  heaped  up  about  its  electrode.  The 
diagrams  in  Figures  7  and  8  will  make  this  clear. 

When  the  current  begins  to  flow,  the  H+  ions  move 
toward  the  right,  and  are  removed  at  the  cathode 
(either  as  gas  or  by  some  secondary  reaction),  and  at 
the  same  time  the  SO4~~  ions  move  toward  the  left, 
and  are  removed  at  the  anode.  The  H+  ion  moves 
five  times  as  fast  as  the  SO4  ion.  By  the  time  six 
SO4 —  ions  have  passed  through  the  electrode,  12 
hydrogen  ions  have  gone  out  of  solution.  10  H+ 
ions  have  in  this  period  of  time  entered  the  region 
about  the  cathode,  and  one  SO4~~  ion  has  entered  the 
region  about  the  anode.  The  region  in  the  center  of 
the  cell  has  not  changed  in  concentration,  but  the 
parts  of  the  cell  on  both  sides  of  it  have  changed, 
and  the  relative  change  has  been  a  large  one. 

It  will  be  seen  at  once  that  the  relative  migration 
velocities  are  inversely  as  the  losses  about  the  electrodes. 
The  cathode  has  lost  one,  the  anode  has  lost  five,  and 
the  migration  velocities  are  as  five  (cation)  to  one 
(anion).  This  means,  too,  that  five  sixths  of  all  the 
electricity  that  has  passed  through  the  cell  has  been 
carried  through  by  the  cation,  and  only  one  sixth  by 
the  anion. 


STORAGE  BATTERIES 


25.  Ionic  Reaction.  —  In  cells  of  the  Daniell  type 
the  ionic  changes  are  very  simple.  A  single  cation 
carries  the  current  toward  the  cathode,  and  leaves 
the  electrolyte  at  that  electrode,  while  a  single  anion 
attends  to  all  the  cell  activities  at  the  anode.  The 
concentration  changes  which  result  from  taking  away 
material  from  the  electrolyte  at  the  cathode  and 
from  adding  it  at  the  anode  are  indicated  in  Figure  9. 

In  our  storage 


FIG.  9. 


Concentration  changes  in  the  Daniell 
cell. 


we   nave  a 
much  more  com- 

plicated system. 
H+  and  SO4" 
do  not  pass  in 
and  out  at  the 
electrodes,  and 
the  really  fun- 
damental cell 
activities  are  cared  for  by  other  ions.  The  ions 
which  are  active  at  the  electrodes  do  not  travel  a 
measurable  distance  into  the  main  body  of  the  elec- 
trolyte. We  must  therefore  expect  two  sets  of  ionic 
reactions  in  a  storage  cell  —  those  between  the  con- 
ducting ions  and  the  active  electrode  ions  and  those 
between  the  active  ions  and  the  substances  of  the 
electrodes.  We  shall  examine  some  possible  and 
plausible  theories  in  Chapter  VIII. 


CHAPTER  IV 
THE    FUNDAMENTAL    CELL    REACTION 

26.  An  active  storage  cell  contains  two  quite 
different  kinds  of  plates  immersed  in  a  rather  strong 
solution  of  sulphuric  acid.  In  storage  battery  par- 
lance one  of  the  plates  is  called  the  "  negative  "  and 
the  other  the  "positive."  In  spite  of  the  fact  that 
the  cell  reaction  is  completely  reversed  each  time  the 
cell  is  charged  and  discharged,  so  that  each  plate  is 
really  positive  half  the  time  and  negative  the  other 
half,  these  terms  are  about  as  good  as  any  that  can 
be  found.  Anode  and  cathode  are  no  more  definite. 
Lead  plate  and  peroxide  plate  could  very  well  be 
used,  and  by  "  the  positive  plate  "  is  meant  the  one 
which  has  lead  peroxide  as  its  chief  constituent. 
The  "  negative "  is  the  one  which  has  as  its  chief 
constituent  spongy,  finely  divided  metallic  lead. 

In  order  to  apply  the  laws  which  we  have  developed 
for  galvanic  cells  in  general  to  the  case  of  the  lead 
accumulator  we  must  first  of  all  know  exactly  what 
chemical  reaction  takes  place  when  current  flows 
through  the  cell. 


40  STORAGE  BATTERIES 

27.  The   Lead   Cell   Reaction.  —  The    complete   re- 
action of  a  lead  accumulator,  working  under  ordinary 
conditions  of  service,  is 

Pb  +  Pb02  +  2  H2SO4  ±£  2  PbSO4  +  2  H20, 

and  the  sign  ^  indicates  that  it  is  perfectly  revers- 
ible. During  discharge  the  reaction  goes  from  left 
to  right.  It  takes  place  of  its  own  accord  and  the 
cell  furnishes  electrical  energy  which  can  be  utilized 
for  work  outside  the  cell.  Under  these  circumstances 
the  sponge  lead  plate  is  the  anode,  —  lead  goes  into 
solution  as  lead  ion,  Pb++,  here,  — and  the  peroxide 
plate  is  the  cathode  —  lead  peroxide  is  reduced  to 
lead  ion  there.  Everywhere  in  the  cell  the  lead  ion 
which  is  produced  finds  SO4  handy,  and  since  lead 
sulphate  is  a  difficultly  soluble  substance,  the  two 
ions  unite  to  form  non-ionic  lead  sulphate,  which  soon 
saturates  the  solution  and  precipitates  in  solid  form. 

28.  Effect  of  High  Current  Density.  —  It  has  been 
said  that  the  reaction  is  completely  reversible  as  long 
as  the  currents  sent  through  the  cell  are  anywhere 
near  the  limits  of   practical   operation.     If   a  very 
large  current  is  sent  through  a  cell  with  very  small 
electrodes,  secondary  effects  appear   in  measurable 
amount.     Persulphates  are  formed  and  some  other 
complex  ions  make  their  appearance. 

Ordinary  Currents.  —  In  ordinary  practice  all  these 
effects  can  be  wholly  neglected.     If  we  are  working 


THE  FUNDAMENTAL   CELL   REACTION          41 

with  a  comparatively  large  cell,  we  can  take  out  the 
electrochemical  unit  of  quantity  of  electricity  with- 
out greatly  changing  the  distribution  of  materials  in 
the  cell,  and  by  the  time  96,540  coulombs  have  been 
sent  through,  %-^i-  gm.  of  lead  have  been  changed 
to  lead  ion  at  the  anode  (the  lead  plate)  and  -2-jp 
gm.  of  lead  peroxide  have  become  lead  ion  at  the 
cathode  (the  peroxide  plate).  At  each  plate  these 
amounts  of  lead  ion  have  found  sulphate  ion  waiting 
for  them  and  equivalent  amounts  of  lead  sulphate 
have  been  precipitated  —  -2-|&  gm.  at  each  plate. 
Nothing  has  yet  been  said  about  the  nature  of  the 
ion  which  travels  back  and  forth  at  the  peroxide 
plate.  Whatever  this  ion  may  be,  it  is  evident  that 
its  decomposition  into  Pb"1"1"  leaves  2  O  behind,  and 
from  the  reaction  it  can  be  seen  that  the  sulphuric 
acid  which  reacts  with  the  lead  ions  furnishes  enough 
hydrogen  to  produce  2  H2O  at  the  positive  plate. 

29.  Reaction  during  Charge.  —  If  now  we  charge 
the  cell,  after  a  period  of  discharge,  we  merely  re- 
verse everything  that  happens  during  discharge. 
The  peroxide  plate  is  now  the  anode.  Here  lead 
ion  goes  out  of  solution  —  leaves  the  ionic  state  —  and 
with  the  aid  of  the  water  in  the  electrolyte  becomes 
lead  peroxide.  At  the  lead  plate,  which  is  now  the 
cathode,  lead  ion  changes  into  metallic  lead,  just  as 
at  any  other  simple  metal-ion  electrode.  At  both 
plates  it  is  the  lead  sulphate  which  furnishes  the 


42  STORAGE  BATTERIES 

constantly  renewed  supply  of  lead  ion  for  the  reaction. 
This  seems  a  little  difficult  at  first  glance,  for  is  not 
lead  sulphate  an  insoluble  substance  ?  If  it  were 
really  insoluble,  of  course  our  cell  could  not  work  in 
this  way,  but  it  is  not.  It  has  a  perfectly  definite 
and  well-known  solubility,  and  while  the  concentra- 
tion of  lead  ion  in  the  solution  is  very  small  indeed, 
it  must  be  remembered  that  the  reservoir  of  lead  sul- 
phate is  very  near  at  hand,  so  that  the  supply  of  lead 
ion  has  only  "  molecular  "  distances  to  travel  to  the 
point  where  it  is  to  be  used. 

30.  Proof  of  the  Formula.  —  This  fundamental  re- 
action has  been  tested  with  the  greatest  care  by  many 
investigators.  There  are  evidently  several  things 
to  be  proven  and  there  are  several  ways  of  proving 
some  of  them. 

What  we  must  know  is  this.  When  we  pass  96,540 
coulombs  through  the  cell  in  the  discharging  direction, 
is  the  result  the  formation  of  £-%$-  gm.  of  lead  sul- 
phate and  -^-  gm.  of  water?  During  this  same 
period  has  the  lead  plate  lost  ^p-  gm.  of  metallic 
lead  and  has  the  peroxide  plate  lost  %-%&  gm.  of  lead 
peroxide?  And  during  the  same  period  has  the 
electrolyte  decreased  its  acid  content  by  1J&  gm. 
of  H2SO4? 

These  points  must  be  proven  for  the  discharge  re- 
action. We  must  also  prove  that  the  reaction  is  per- 
fectly reversible  and  that  during  charge  exactly  the 


THE  FUNDAMENTAL   CELL  EE ACTION 


43 


same  amounts  of  exactly  the  same  materials  react, 
and  no  others,  the  reaction  being  now  from  right 
to  left. 

The  change  in  the  content  of  lead,  lead  peroxide, 
and  lead  sulphate  in   the  plates  must  be  found  by 


X 

^ 

)  PEROXIDE 

528* 

^ 

^ 

^ 

^ 

^ 

^ 

"^^ 

^ 

^ 

^ 

•< 

^ 

R  ?  g  5 

3V3T  dO  3DVJ.N3OU 

^ 

-^**" 

"^ 

^ 

^^ 

^ 

^ 

^ 

^^ 

"^ 

8" 

1         t         34         56         7         6        9        10       II        14       13       14       15 

AMPERE-HOURS 


FIG.  10.  —  Change  in  the  PbO2  content  of  the  peroxide  plate  during 
charge  and  discharge. 

careful  chemical  analysis  of  plates  after  various  times 
of  charge  and  discharge. 

Figure  10  shows  the  results  obtained  by  analyzing 
the  active  material  of  the  positive  plate  after  various 
times  of  charge  and  discharge.  It  will  be  seen  that 
the  content  of  the  plate  in  peroxide  is  accurately  pro- 
portional to  the  amount  of  electricity  which  has 
passed  through  the  cell,  just  as  required  by  our  fun- 


44 


STORAGE  BATTERIES 


1.15 


damental  reaction.  Similar  analyses  of  the  active 
material  of  the  negative  plate  show  similar  curves 
for  the  lead  content,  and  the  lead  sulphate  content  has 
been  found  to  be  an  equally  good  indication  of  the 
condition  of  the  cell  as  to  charge  or  discharge. 

The  easiest  of 
all  the  changes 
to  follow  is  that 
in  the  electrolyte. 
Here  we  can  fol- 
low the  change  of 
concentration  by 
merely  measur- 
ing the  density 
of  the  acid  from 
time  to  time. 
This  is  shown  in 
Figure  11.  Evi- 
dently there  will 
be  a  lag  of  density  behind  the  value  properly  belong- 
ing to  any  given  time  after  charge  or  discharge  has 
begun.  For  the  acid  is  being  formed  or  used  up  in- 
side the  plate,  and  must  diffuse  in  or  out  as  the  re- 
action goes  on.  This  is  a  comparatively  slow  process, 
and  we  must  therefore  expect  that  just  at  the  begin- 
ning of  either  charge  or  discharge  the  acid  density 
will  remain  constant,  even  though  some  current  has 
passed.  The  curves  of  Figure  12  are  for  the  very 


10  20  30  40  50 

AMPERE-HOURS  OF  CHARGE  AND  DISCHARGE 

FIG.  11.  —  Change  in   acid  density  during 
charge  and  discharge. 


THE  FUNDAMENTAL   CELL  REACTION 


45 


beginning  of  charge  and  discharge,  and  they  show 
this  lag  effect  very  clearly.  These  are  really  pieces 
which  belong  at  the  beginning  of  the  curves  of  Figure 
11,  but  they  would  not  show  if  plotted  in  the  time 
units  of  that  figure.  In  their  own 
diagram  the  time  axis  is  greatly 
drawn  out  to  show  the  effect  more 
clearly. 

When  we  have  once  decided  that 
this  fundamental  reaction  really 
represents  what  happens  in  a  lead 
accumulator  during  its  practical 
operation,  we  have  made  a  great 
step,  and  with  the  aid  of  the  gen- 
eral theory  developed  in  earlier 
chapters  we  can  go  a  long  way 


AMPERE-HOURS 


toward  explaining  the  effect  of  va-  FIG.  12.  — First  part 

f  .-,  ,!  ofcurvesofFig.il. 

riOUS  factors  on  the  Cell.  Enlarged  scale. 

It  has  taken  a  long  time  to  gather 
the  evidence  which  proves  the  correctness  of  our  fun- 
damental cell  reaction,  and  there  are  probably  a  good 
many  storage  battery  experts  who  still  feel  doubtful 
as  to  its  completeness.  Many  of  them  have  wished 
to  introduce  intermediate  steps,  such  as  the  forma- 
tion of  lead  persulphate  or  persulphuric  acid  at  the 
peroxide  plate  during  charge.  It  is  evident  that  as 
long  as  the  processes  assumed  are  reversible  and  lead 
to  the  same  final  formula  as  the  one  we  are  using,  any 


46  STORAGE  BATTERIES 

number  of  intermediate  reactions  could  be  assumed 
without  affecting  the  validity  of  our  reaction  in  the 
least.  But  even  this  opportunity  for  introducing 
hypotheses  and  analogies  is  removed  when  we  ex- 
amine the  electromotive  force  equations  for  the  cell, 
which  we  shall  take  up  in  a  future  chapter.  When 
all  the  evidence  is  taken  into  consideration,  our  fun- 
damental reaction  seems  to  be  proven. 


CHAPTER   V 

THE  ACTIVE  IONS 

31.  It  does  not  take  any  training  in  theoretical 
science  to  make  it  quite  clear  that  the  actual  carry- 
ing of  current  through  the  storage  cell  is  done  by 
the  sulphuric  acid,  and  we  can  be  very  sure  that  it  is 
done  by  the  ions  H+  and  SO4~~.  Both  lead  and 
lead  peroxide  are  so  very  slightly  soluble  in  sulphuric 
acid  that  their  presence  in  the  electrolyte  can  hardly 
be  shown  by  analytical  means.  The  concentration 
of  the  ions  which  pass  back  and  forth  at  the  elec- 
trodes must  always  be  exceedingly  minute,  and  this 
small  amount  of  ion  cannot  have  the  least  relation 
to  the  huge  current  that  can  be  sent  through  a  large 
storage  cell. 

In  this  respect  the  storage  cell  differs  from  most 
galvanic  cells.  And  it  is  precisely  in  this  very  point 
that  the  remarkable  properties  of  the  lead  cell  as 
an  accumulator  are  all  bound  up.  If  the  ion  of  the 
electrodes  reached  any  large  concentration,  we  would 
have  all  the  difficulties  in  the  way  of  trees  and  short 
circuits  which  appear  in  most  cells  when  we  try  to 
reverse  them  and  use  them  as  accumulators.  The 

47 


48  STORAGE  BATTERIES 

active  material  would  soften  and  move  all  about  the 
cell,  growing  at  the  favored  points  and  not  at  the 
others.  In  the  lead  cell  material  produced  during 
either  charge  or  discharge  is  deposited  "right  in  its 
tracks,"  to  use  a  homely  expression,  and  the  plates 
preserve  their  condition. 

32.  What  Ions  carry  Current?  —  But  if  the  current 
is  all  carried  by  ions  which  do  not  pass  back  and 
forth  at  the  electrodes,  there  must  somewhere  in  the 
cell  be  a  loading  and  unloading  of  electricity  from 
ion  to  ion,  and  the  complete  expression  for  the  cell 
reaction  should  show  this  transfer.     As  a  matter  of 
fact  it   cannot  be   shown  by  any   purely   chemical 
means,  nor  is  it  at  all  necessary  to  try.    The  reaction 
we  have  adopted  is  the  necessary  and  complete  ex- 
pression for  everything  that  takes  place  in  the  cell, 
from  a  merely  chemical  point  of  view.     We  can  get 
some  theories  which  fit  the  facts  pretty  well,  and 
it  will  be  seen  a  little  later  that  these  theories  are 
subject  to  rather  severe  tests  of  a  quantitative  sort. 
At  any  rate,  it  is  always  interesting  to  develop  the 
possible  theories  for  such  a  chemically  unattackable 
problem,  and  so  we  will  examine  one  of  the  most 
plausible. 

33.  At  the  Negative  Plate.  —  Let  us  start  with  the 
negative  plate.     During  discharge  this  is  the  anode 
of  the  cell.     The  acid  is  doing  the  carrying  of  current 
through  the  cell,  and  SO4      ion  is  therefore  moving 


THE  ACTIVE  IONS  49 

toward  the  anode.  The  electrode  is  probably  re- 
versible with  respect  to  Pb++  ion,  and  lead  goes  into 
solution  as  Pb++  in  proportion  to  the  amount  of  cur- 
rent which  passes  through  the  electrode.  It  never 
gets  far,  for  the  SO4~~  is  moving  toward  it,  even  if 
there  were  not  enough  in  the  electrolyte,  and  lead 
sulphate  is  precipitated  in  the  very  spot  where  the 
lead  ion  was  formed  from  the  metal.  The  only 
thing  that  is  left  over  after  this  reaction  has  been 
completed  is  hydrogen  ion,  H+,  and  this  is  doing  the 
carrying  of  current  through  the  electrolyte  toward 
the  cathode,  —  in  this  case  the  peroxide  plate.  If 
we  can  take  this  extra  H+  into  our  reaction  at  the 
cathode,  we  will  be  able  to  reach  a  balance,  and  our 
theory  will  at  least  be  a  possible  one. 

Leaving  aside  for  the  moment  the  matter  of  the 
ions,  we  can  say  with  certainty  :  — 

Sulphuric  acid  carries  the  current  across  the  space 
from  plate  to  plate.  The  acid  is  separated  during 
this  time  into  2  H  and  SO4. 

For  discharge 

Pb  +  SO4  ->  PbSO4. 
Pb02  +  H2  +  H2S04  ->  PbS04  +  2  H2O. 
In  sum 

Pb  +  Pb02  +  2  H2S04  ->  2  PbS04  +  2  H2O. 

34.  At  the  Peroxide  Plate.  —  It  does  not  require  a 
very  vivid  scientific  imagination  to  discover  a  simple 


50  STORAGE  BATTERIES 

and   reversible   reaction   which   takes   in   the   ionic 
change  at  the  lead  plate. 

Pb  _>  Pb++. 

Metal 


Solid 

For  the  peroxide  plate  we  need  a  more  complicated 
set  of  changes,  and  Liebenow  has  suggested  an  ion 
which  fits  the  facts  very  well  indeed.  Suppose  the 
peroxide  plate  to  be  reversible  with  respect  to  the 
PbO2  —  ion.  We  then  have  at  this  plate  during 
discharge 

PbO2  -»  PbO2-~, 

Solid 

PbO—  +  4  H+  ->  Pb+^  +  2  HO, 


Solid 

and  if  we  add  the  reactions  at  the  lead  and  lead 
peroxide  plates,  we  get 

Pb  +  Pb02  +  2  S04~  +  4  H+  *£  2  PbS04  +  2  H2O, 

Metal          Solid  Solid     ' 

which  is  our  fundamental  reaction 

Pb  +  Pb02  +  2  H2SO4  ±£  2  PbS04  +  2  H2O. 

This  is  completely  reversible,  and  it  will  also  be 
found  that  our  separate  ionic  reactions  represent 
completely  reversible  changes. 

35.  Diagrams  of  Charge  and  Discharge.  —  The  accom- 
panying diagrams  may  make  all  this  still  clearer. 
The  cell  is  discharging  —  it  is  furnishing  current 


THE  ACTIVE  IONS  51 

for  use  in  the  external  circuit.  The  current  is  flow- 
ing into  the  cell  at  the  lead  plate,  which  is  therefore 
the  anode.  Here  metallic  lead  passes  through  the 
electrode  (Fig.  13)  and  changes  into  lead  ion,  Pb++, 
carrying  96,540  coulombs  with  it  for  each  2-^1  gm.  of 
lead  that  go  into  solution.  The  lead  ion  has  hardly 
passed  the  electrode  before  it  meets  with  SO4  in 
the  electrolyte  (Fig.  14).  Lead  sulphate  being  so 
slightly  soluble,  it  requires  only  a  very  small  concen- 
tration of  lead  ion  and  sulphate  ion  in  solution  to 
reach  the  limit  of  solubility  of  lead  sulphate.  This 
substance  is  therefore  formed  from  the  two  ions  as  a 
solid,  and  removed  from  the  electrolyte  as  fast  as  it 
is  produced. 

36.  Discharge.  — On  discharge  (see  Figure  14)  the 
lead  peroxide  plate  is  the  cathode.  It  is  certainly 
reversible  with  respect  to  some  ion,  and  PbO2 —  seems 
to  fit  the  necessary  conditions.  This  PbO2  is  con- 
stantly formed  from  the  solid  PbO2  of  the  plate,  just 
as  Pb++  is  formed  from  the  solid  lead  of  the  anode. 
It  starts  toward  the  anode,  being  an  anion,  as  its 
two  —  signs  indicate.  Before  it  has  more  than 
passed  the  electrode  it  meets  with  H+,  of  which 
there  is  always  plenty  about  in  a  concentrated  sul- 
phuric solution,  even  if  it  were  not  moving  toward 
the  cathode  carrying  the  current.  It  reacts  with 
this  H+,  forming  Pb++  and  water  (Fig.  15),  and  the 
Pb++,  finding  SO4  in  plenty,  soon  saturates  the 


©©©© 


FIG.  13.  —  The  begin- 
ning of  discharge. 


8  < 


5 


UJ 


O 

Ld 


FIG.  15.  — The  third 
stage  in  the  dis- 
charge reaction. 


n 


FIG.  14.  —  The  second 
stage  in  the  discharge 
reaction. 


n 


FIG.  16.  —  Discharge  com- 
plete. 


THE  ACTIVE  IONS  53 

solution  with  lead  sulphate,  which  is  precipitated 
very  nearly  in  the  spot  from  which  the  peroxide 
started  (Fig.  16). 

It  will  do  no  harm  to  go  over  the  changes  in  the 
reverse  direction,  just  to  fix  the  whole  reaction  more 
firmly  in  our  minds. 

Charge.  —  The  cell  is  charging  (see  Figure  17) .  The 
peroxide  plate  is  now  the  anode,  and  contains  a  con- 
siderable proportion  of  finely  divided  lead  sulphate 
from  the  previous  discharge.  Pb++  and  SO4  are 
formed  as  fast  as  they  are  needed  from  this  reser- 
voir in  the  plate,  and  the  Pb++  reacts  with  the  water 
of  the  electrolyte,  forming  H+  and  PbO2  (Fig. 
18).  The  PbO2~  passes  through  the  electrode  (Fig. 
19)  and  is  deposited  as  solid  PbO2  very  close  to  the 
point  where  lead  sulphate  went  into  solution.  H+  and 
SO4  are  left  in  the  electrolyte  in  proportion  to  the 
amount  of  current  which  has  passed  (Fig.  20). 

The  lead  plate  is  cathode  during  charge.  Here 
also  there  is  a  reservoir  of  fine  lead  sulphate  from 
the  previous  discharge.  This  furnishes  a  constant 
supply  of  Pb++  and  SO4~~,  and  the  electrode  is  re- 
versible-with  respect  to  Pb++.  So  Pb++  passes  out 
and  changes  to  metallic  lead,  sending  a  correspond- 
ing quantity  of  electricity  along  through  the  ex- 
ternal circuit,  while  the  SO4~~  finds  itself  moving 
toward  the  anode.  It  will  find  its  equivalent  of 
H+  in  the  solution,  and  our  equations  show  that 


E 


II  ge 

~*H5  3i 

5n  o° 

^S  <t 

H  LJ  5, 

m  li^ 


Fia.  17. —  The  beginning  of  charge.  FIG.  18.  —  Second  stage  of  the  charge  re- 

action. 


/ 
/     _T1 

\ 


-N  1 1  i  ft 

>nr}|  ^hH 


Fio.  19.  —  Third  stage  in  charge  reaction.  FIG.  20.  —  Charge  complete. 


THE  ACTIVE  IONS  55 

acid  is  produced  during  charge  in  proportion  to  the 
amount  of  material  reacting,  and  that  it  is  used  up 
in  the  same  proportion  during  discharge.  It  also 
expresses  everything  else  that  is  contained  in  our 
fundamental  reaction,  and  gives  us  at  least  a  pos- 
sible picture  of  what  takes  place  at  the  electrodes 
as  well.  We  have  shown  that  it  is  quite  possible 
to  have  all  the  current  carried  through  the  cell  from 
plate  to  plate  by  the  ions  of  the  acid,  provided  these 
two  ions  react  near  the  electrodes  to  produce  ions 
like  the  ones  we  have  assumed.  Our  electrode 
reactions  are  perfectly  reasonable  ones,  and  are,  as 
matter  of  fact,  supported  by  a  great  deal  more  evi- 
dence than  we  can  yet  call  to  their  support.  We 
shall  return  to  them  in  a  later  chapter. 


CHAPTER  VI 

SOME  PERTINENT  PHYSICAL  QUERIES 

37.  A  host  of  questions  arises  even  at  this  early 
point   in   the    discussion   of   the   lead   storage   cell. 
Even  if  we  suppose  that  we  have  satisfactorily  dis- 
posed of  the  chemical  changes,  and  found  a  pair  of 
ions  that  might  do  the  work  at  the  electrodes,  how 
can  we  explain  a  good  many  things  about  the  pe- 
culiar nature  of  the  materials  of  the  cell  ? 

Premises.  —  These  questions  can  best  be  discussed 
if  the  reader  will  keep  in  mind :  — 

(I)  The  ions  which  pass  back  and  forth  at  the 
electrodes  have  only  molecular  distances  to  travel. 

(II)  The   particles  of   active   material   are  very 
small  indeed. 

(Ill)  The  active  materials:  —  lead,  lead  peroxide, 
and  lead  sulphate  are  all  very  slightly  soluble  in  con- 
centrated sulphuric  acid. 

38.  Queries  and  their  Answers.  —  QUERY  1.     How 
can  storage  plates  keep  their  shape?     How  does  it 
happen  that  a  battery  can  be  sent  through  thousands 
of  charges  and  discharges  without  much  growth  of 
trees  or  sponge  ? 

66 


SOME  PERTINENT  PHYSICAL   QUERIES         57 

Just  because  all  the  solid  substances  concerned 
are  so  very  slightly  soluble  in  the  electrolyte.  The 
ion  which  passes  back  and  forth  at  the  electrode  has 
no  chance  to  wander  far  enough  to  deposit  at  even 
ka  measurable  distance  from  its  point  of  origin. 
SO4  is  everywhere  waiting  for  the  Pb++,  and  in- 
soluble PbSO4  is  precipitated  almost  instantly.  This 
is  one  of  the  prime  secrets  of  the  success  of  the  lead 
cell,  and  the  main  reason  why  its  plates  preserve 
their  mechanical  structure  as  well  as  they  do.  In 
another  sense  it  is  a  disadvantage,  for  it  means  that 
the  particles  of  active  material  will  be  exceeding  fine 
and  small,  and  that  there  will  not  be  much  inter- 
growth  and  interlocking  between  neighboring  par- 
ticles. In  the  ideal  cell  both  extreme  insolubility 
and  intergrowth  of  particles  might  occur  simul- 
taneously, but  not  in  practice. 

QUERY  2.  The  lead  peroxide  of  the  positive  plate 
is  in  contact  with  a  lead  support.  Why  does  not 
the  plate  discharge  of  its  own  accord?  Does  it  not 
contain  all  the  necessary  substances  for  the  reaction 

Pb  +  Pb02  +  2  H2S04  ->  2  PbS04  +  2  H20  ? 

It  does ;  and  self-discharge  always  takes  place 
when  a  peroxide  plate  is  standing  fully  charged. 
But  before  it  has  gone  far  all  the  finely  divided 
rough  lead  on  the  surface  of  the  lead  support  has 
reacted  and  then  the  plate  is  protected  by  its  dense 


58  STORAGE  BATTERIES 

layer  of  lead  sulphate,  just  as  a  lead  plate  protects 
itself  in  sulphuric  acid. 

If  the  surface  of  the  lead  support  is  roughened  or 
increased,  the  action  will  be  stronger,  and  Plante 
plates  were  originally  formed  for  service  by  means 
of  this  very  action.  Our  modern  plates  have  a  very 
much  greater  proportion  of  active  material  to  surface 
of  lead  support,  and  therefore  the  loss  of  energy  due 
to  this  "local  action"  is  a  comparatively  small  one. 
(See  page  182.) 

QUERY  3.  How  does  it  happen  that  a  lead  accu- 
mulator with  a  difference  of  potential  of  two  volts 
between  its  plates  can  stand  on  open  circuit  without 
immediately  discharging  itself?  Under  proper  con- 
ditions water  (made  acid  with  sulphuric  acid)  can 
be  completely  decomposed  into  hydrogen  and  oxy- 
gen at  1.5  or  1.6  volts.  Why  does  not  our  cell 
begin  to  decompose  its  electrolyte  and  keep  on  form- 
ing gas  until  the  plates  are  quite  discharged? 

Because  the  plates  of  our  cell  are  made  of  lead  and 
lead  peroxide.  There  is  a  great  difference  in  the 
amount  of  work  required  to  form  bubbles  of  hydro- 
gen rapidly  on  surfaces  of  various  metals.  It  takes 
2.5  or  2.6  volts  to  cause  gas  to  form  rapidly  in  a 
lead  accumulator,  and  at  1.6  volts  —  the  electromo- 
tive force  at  which  gas  forms  on  platinum  electrodes 
—  hydrogen  forms  bubbles  so  slowly  on  a  lead  sur- 
face that  losses  due  to  this  cause  are  quite  negligible. 


SOME  PERTINENT  PHYSICAL   QUERIES         59 

Even  at  2  volts  the  evolution  of  hydrogen  is  so  slow 
as  to  be  immeasurable.  (See  page  217  for  the  effect 
of  impurities.) 

QUERY  4.  How  can  it  be  that  lead  sulphate  is 
formed  during  the  discharge  of  our  cell,  and  how 
can  this  substance  change  back  so  readily  to  lead 
and  lead  peroxide  ?  Is  not  "  sulphation  "  the  most 
dangerous  disease  that  can  come  upon  a  battery  ? 

The  explanation  is  a  matter  of  surface,  like  so  many 
others  in  this  subject.  The  lead  sulphate  which 
forms  in  the  plate  during  a  healthy  discharge 
differs  greatly  in  size  of  grain  from  the  same  sub- 
stance taken  from  the  bottle  on  the  laboratory  shelf, 
and  just  as  much  from  the  material  which  causes  what 
is  called  in  battery  parlance  "sulphation."  If  ordi- 
nary commercial  lead  sulphate  be  made  into  a  paste 
and  filled  into  a  lead  support,  it  does  not  change  to 
lead  at  the  cathode  and  lead  peroxide  at  the  anode 
easily.  It  can  be  subjected  to  the  action  of  the  cur- 
rent for  a  very  long  time  without  being  completely 
transformed,  arid  it  never  does  make  a  good  coherent 
plate.  When  a  cell  is  allowed  to  stand  discharged 
for  many  weeks  the  fine  grains  of  sulphate  which 
are  formed  during  normal  discharge  suffer  an  inter- 
esting change.  True  crystallization  begins  on  the 
larger  particles,  and  the  substance  goes  into  solution 
at  the  small  ones.  It  moves  through  the  solution 
and  continues  to  deposit  on  the  large  grains  until 


60  STORAGE  BATTERIES 

the  small  grains  have  completely  dissolved  and  the 
large  ones,  fewer  in  number,  have  grown  to  consider- 
able size.  The  plate  is  now  sulphated,  and  if  it  is 
charged  for  the  ordinary  time,  it  by  no  means  returns 
to  its  original  condition  of  healthy  charge.  The  large 
crystals  of  sulphate  do  not  go  into  solution  com- 
pletely. In  fact,  they  hardly  dissolve  at  all,  arid 
long  before  the  cell  has  been  brought  back  to  its 
charged  state  reaction  has  ceased  and  the  current  is 
merely  producing  gas.  It  is  possible  to  restore  a 
sulphated  cell,  but  the  charge  must  be  continued  so 
long  that  gassing  breaks  up  the  active  material,  and 
even  when  the  remaining  sulphate  has  all  been  forced 
to  react,  a  large  part  of  the  original  capacity  of  the 
cell  has  been  lost.  (See  page  216.) 

QUERY  5.  Metallic  lead  in  the  form  of  a  bar  or 
plate  is  not  dissolved  by  sulphuric  acid  under  ordi- 
nary circumstances,  and  this  is  especially  true  of  acid 
of  the  concentration  used  in  storage  batteries.  The 
grids  of  paste  plates  and  the  main  body  of  Plante 
plates  resist  the  attack  cf  the  acid  during  the  whole 
life  of  the  plates.  Lead  is  one  of  the  metals  which 
"protects  itself"  from  solution  in  reagents  by  the 
formation  of  a  dense  layer  of  slightly  soluble  material 
on  the  surface.  It  is  a  familiar  fact  that  lead  pipes 
cannot  be  used  for  pure  distilled  water  without 
danger  of  contamination,  for  in  this  case  the  sub- 
stance formed  is  not  dense  and  does  not  protect  the 


SOME  PERTINENT  PHYSICAL   QUERIES         61 

metal.  The  hydroxide  which  forms  under  these  cir- 
cumstances is  fluffy  and  breaks  away  from  the  sur- 
face, and  the  plate  rapidly  dissolves.  But  if  the 
water  passing  through  the  pipe  is  not  pure, —  if  it 
contains  carbonates,  chlorides,  and  sulphates  even  in 
small  amounts,  —  dense  protecting  coatings  of  carbon- 
ate, chloride,  or  sulphate  are  formed  and  the  metal 
is  no  longer  dissolved.  It  is  safe  enough  to  use  lead 
pipes  for  ordinary  water  even  if  it  is  to  be  used  for 
drinking  purposes. 

How  is  it,  then,  that  the  lead  of  the  negative  plate 
can  pass  easily  and  rapidly  into  the  form  of  lead 
ion?  Why  do  not  the  particles  of  lead  so  protect 
themselves  and  refuse  to  react?  And  if  because  of 
their  very  fineness  the  protecting  layer  which  might 
be  formed  makes  up  a  considerable  part  of  the  whole 
bulk  of  the  grains,  why  does  not  the  self-discharge 
necessary  to  produce  this  protecting  layer  greatly 
reduce  the  activity  of  the  lead  plate? 

While  it  is  quite  true  that  the  particles  of  lead  on 
the  negative  plate  are  very  small,  they  are  still  quite 
large  in  comparison  with  the  protecting  layer  of  sul- 
phate which  is  sufficient  to  prevent  farther  action. 
At  the  end  of  charge  a  part  of  the  energy  is  lost  by 
formation  of  sulphate  at  the  lead  plate,  but  in  prac- 
tice it  is  a  very  small  fraction  of  the  whole.  But 
when  current  is  passing  through  the  cell  in  the  dis- 
charge direction  a  very  different  state  of  things  pre- 


62 


STORAGE  BATTERIES 


vails.  Suppose  our  cell  to  be  first  on  open  circuit 
and  that  we  are  looking  at  what  happens  at  the  lead 
plate  and  able  to  see  everything  that  occurs.  Lead 

changes  to  lead  ion, 
Pb++,  and  this  goes 
into  solution,  leaving 
the  plate  negatively 
charged.  The  Pb++ 
finds  SO4 waiting 
and  precipitates  as  in- 
soluble PbSO4,  but  it 
leaves  2  H+  behind  it, 
and  the  condition  of 
strain  set  up  by  the 
positively  charged  ion 
in  the  electrolyte  and 
the  negatively  charged 
plate  is  not  relieved 
(Figure  21).  It  only 
takes  the  presence  of 
a  very  small  concen- 

Fia.  21.  —  Electrostatic    equilibrium    tratlon  of  ion  in  solu- 
about  a  lead  plate.  ,  • 

tion  to  set  up  an  at- 
traction so  strong  that  no  more  ion  leaves  the  plate. 
The  electrode  is  in  equilibrium  with  respect  to  Pb++. 
It  has  protected  itself  sufficiently  by  sacrificing  a  very 
minute  fraction  of  its  whole  mass. 

But  as  soon  as  the  external  circuit  is  closed  and 


LU 


SOME  PERTINENT  PHYSICAL   QUERIES         63 

current  begins  to  pass,  the  H+  is  no  longer  bound  by 
an  electrostatic  attraction.  The  lead  plate  can  dis- 
charge itself  through  the  wires  and  the  H+  can  pro- 
ceed on  its  way  toward  the  cathode,  carrying  its 
equivalent  of  electricity  with  it.  The  electrode  is 
no  longer  in  equilibrium,  and  more  lead  goes  into 
solution,  becomes  Pb++,  reacts  with  SO4  ,  and  frees 
more  H+.  This  continues  as  long  as  current  is 
being  taken  from  the  cell. 


CHAPTER  VII 

ENERGY  RELATIONS 

39.  Any  arrangement  whatever  which  runs  of  its 
own  accord  and  which  can  furnish  energy  for  doing 
outside  work  as  well  must  draw  upon  some  store  for 
the  energy  expended.     A  charged  storage  cell  con- 
tains potential  chemical  energy.     It  differs  in  no  way 
from  any  other  galvanic  cell  in  this,  and  if  we  knew 
of  practical  ways  of  manufacturing  lead  sponge  and 
lead  peroxide  of  exactly  the  same  physical  character- 
istics as  those  possessed  by  the  active  materials  of 
our  charged  accumulator,  we  could  build  a  cell  just 
like  it  in  every  way  without  any  charging  process 
whatever.       It  merely  happens  that   the  very  best 
way  of  manufacturing  lead  sponge  and  lead  peroxide 
of  exactly  the  right  quality  is  to  pass  a  current  of 
electricity  through  a  discharged  storage  cell.     The 
materials  themselves  are  no  more  electrical  than  the 
same  substances  in  bottles  on  the  laboratory  shelf. 

40.  Transformations  of  Energy.  —  There  is  hardly  a 
branch  of  science  where  we  can  be  so  sure  of  our 
footing  as  in  calculations  which  involve  the  trans- 
formation of  quantities  of  energy  from  one  form  to 

64 


ENERGY  RELATIONS  65 

another,  especially  in  the  calculation  of  reversible 
changes,  and  it  is  difficult  to  imagine  any  arrange- 
ment which  could  be  more  perfectly  reversible  than  a 
storage  cell.  Small  losses  occur  even  in  a  big  storage 
cell.  Some  gas  escapes  and  cannot  be  taken  into 
our  calculation,  and  there  is  some  local  action  at  the 
plates  with  corresponding  evolution  of  a  little  heat. 
But  the  same  is  true  in  any  arrangement  known  to 
man,  and  in  most  cases  the  losses  are  very  much 
greater  than  in  our  cell. 

Electrochemical  Reaction.  —  We  can  apply  the  law 
of  the  Conservation  of  Energy.  Applied  to  our 
own  particular  case  this  law  says :  If  we  have  at  our 
disposal  a  system,  represented  by 

Pb  +  Pb02  +  2  H2S04 

and  consisting  of  207  gm.  of  lead,  239  gm.  of  lead 
peroxide,  and  196  gm.  of  sulphuric  acid,  and  this 
system  changes  of  its  own  accord  into  another 

2  PbS04  +  2  H20 

consisting  of  606  gm.  of  lead  sulphate  and  36  gm.  of 
water,  a  definite  and  determined  amount  of  energy 
will  be  set  free,  which  can  be  utilized  for  doing 
work.  If  the  reaction  is  perfectly  reversible  and  no 
energy  has  managed  to  get  away  from  us,  we  can 
restore  the  original  condition  of  the  system  by  ex- 
pending the  same  quantity  of  energy  on  it. 


66  STORAGE  BATTERIES 

Our  own  special  interest  lies  in  a  chemical  reaction, 
but  the  same  law  applies  for  any  change  whatever. 
The  original  condition  might  be  represented  by  a 
certain  mass  of  water  at  the  top  of  a  dam  and  the 
final  condition  by  the  same  mass  at  the  bottom. 
Here  we  would  have  no  difficulty  in  calculating  the 
quantity  of  work  obtainable  by  the  fall  of  the  water, 
and  the  same  amount  of  work  would  carry  it  back 
to  the  top,  provided  all  our  machines  were  friction- 
less  and  worked  with  100  %  efficiency. 

41.  Thermochemical  Reaction.  —  Now  for  the  next 
step.     If  we  should  take  the  amounts  of  the  various 
materials  on  the  left  side  of  our  fundamental  equa- 
tion, and  should  mix  them  all  up  into  a  pasty  mass, 
we  would  not  get  any  electrical  current  from  it,  but 
we  would   get   a   definite  amount  of   heat  set  free. 
We  will  get  the  same  total  amount  of  energy  from 
the  reaction  in  either  case,  provided  our  cell   does 
not  itself  heat  up  or  cool  down  during  the  reaction 
of  these  amounts  of  its  materials.     In  the  one  case 
we  should  measure  the  amount  of  available  energy 
in  heat  units,  or  calories,  and  a  calorie  is  the  amount 
of  heat  required  to  raise  the  temperature  of  1  gm.  of 
water  1°  C.     In  the  other  case  we  should  measure 
the  amount  of  available  energy  in  electrical   units, 
joules  (volt-coulombs). 

42.  Heat  Changes  in  the  Cell.  — If  our  cell  does  heat 
up  while  it  is  sending  out  its  96,540  coulombs,  we 


ENERGY  RELATIONS  67 

must  remember  the  amount  of  heat  which  appears  in 
this  way,  and  we  must  expect  to  get  less  energy 
from  the  cell  for  use  in  the  external  circuit  if  a  part 
of  the  total  energy  of  the  reaction  has  been  used  to 
heat  the  air  of  the  room.  If  the  cell  cools  while  it 
is  working,  we  might  expect  to  get  more  than  the 
calculated  amount  of  energy,  and  to  this  point  we 
will  come  back  later. 

But  if  the  cell  neither  heats  nor  cools  during  the 
passage  of  96,540  coulombs,  the  law  of  the  Conser- 
vation of  Energy  gives  us  our 

First  Fundamental  Equation 

chemical  energy  expanded  =  electrical  energy  produced. 

Before  we  can  go  any  farther  we  must  know  the 
numerical  factor  for  transforming  joules  to  calories 
(or  vice  versa),  and  this  has  been  often  determined. 
It  takes  4.18  joules  to  raise  the  temperature  of  1  gm. 
of  water  1°  C. 

The  determination  of  the  heat  of  the  reaction 

Pb  +  PbO2  +  2  H2SO4;£2  PbSO4  +  2  H2O 

cannot  be  carried  out  directly  with  accuracy  because 
of  the  slowness  of  the  reaction  when  the  substances 
are  mixed  up  together.  It  can  only  be  determined 
by  indirect  measurement,  and  the  best  results  have 
been  obtained  by  using  very  dilute  sulphuric  acid. 

Applying  a  correction  to  be  explained  immediately, 
the  heat  of  this  reaction  for  acid  of  density  1.044 


68  STORAGE  BATTERIES 

(0.70  gm.-mol.   H2SO4  per   liter  of  electrolyte)  is 
87,000  calories.     A  cell  containing  acid  of  this  den- 
sity neither  heats  nor  cools  while  it  is  working. 
Now  see  how  simple  our  calculation  becomes : 

87,000  calories  x  4.18  =  364,000  joules, 

and  this  is  the  amount  of  electrical  energy  which  be- 
comes available  when  207  gm.  of  lead  and  239  gm. 
of  lead  peroxide  have  reacted  with  196  gm.  of  sul- 
phuric acid  (in  rather  dilute  solution)  to  produce 
606  gm.  of  lead  sulphate  and  36  gm.  of  water. 

If  we  arrange  things  so  that  the  reaction  can  take 
place  in  a  galvanic  cell,  2  x  96,540  coulombs  will  pass 
through  the  cell  by  the  time  these  amounts  have 
reacted.  These  193,080  coulombs  will  have  given  us 
364,000  joules  of  work,  and  the  voltage  of  the  cell 
must  therefore  be 

364,000  volt-coulombs  _  i  QQ      u 
2  x  96,540  coulombs 

This  agrees  closely  with  the  measured  voltage  of  a 
cell  containing  this  rather  dilute  acid  as  electrolyte. 
While  there  is  no  doubt  whatever  about  the  cor- 
rectness of  this  principle,  there  is  often  a  great  deal 
of  difficulty  in  obtaining  accurate  data  on  the  heats 
of  reaction.  In  this  case  a  number  of  reactions  had 
to  be  used,  and  the  final  result  calculated  in  a  round- 
about way  by  eliminating  the  heats  of  the  various 


ENERGY  RELATIONS  69 

intermediate  steps.  Even  in  this  case  there  is  no 
doubt  as  to  the  correctness  of  the  method,  but  the 
final  result  is  always  afflicted  with  a  large  experi- 
mental error. 

43.  Heating  and  Cooling  of  the  Cell.  —  The  ordinary 
practical  storage  cell  contains  acid  of  density  about 
1.210.     It  cools  during  discharge  and  heats  during 
charge,  and  can  therefore  not  be  brought  under  the 
simple  law  we  have  just  used.     We  can  make  some 
qualitative  statements  about  it,  however. 

Since  it  cools  during  discharge,  it  must  take  into 
its  system  a  certain  amount  of  heat  from  the  room 
during  the  passage  of  96,540  coulombs.  At  least  a 
part  of  this^  heat  will  be  transformed  into  electrical 
energy.  Since  we  always  calculate  on  the  basis  of 
96,540  coulombs,  the  voltage  of  this  cell  must  be 
higher  than  it  would  be  if  it  did  not  cool  down  while 
it  was  working. 

During  charge,  the  cell  gets  hotter  than  the  room. 
A  part  of  the  energy  supplied  to  charge  it  is  used  in 
heating  the  surrounding  objects,  and  it  therefore 
takes  more  energy  to  completely  reverse  the  reaction 
than  it  would  if  the  cell  did  not  change  its  tempera- 
ture during  charge.  Since  we  use  the  same  96,540 
coulombs  for  the  reversal,  the  charging  voltage  must 
be  higher  than  it  would  be  if  the  cell  did  not  heat  up. 

44.  The  General  Equation.  —  We  can   handle   this 
case  quantitatively  just  as  easily  as  the  simple  previ- 


70  STORAGE  BATTERIES 

ous  one,  for  we  have  what  is  called  the  Second  Law 
of  Thermodynamics,  which  states 


For  our  case 

W  '—  available  electrical  energy. 
Q  =  heat  of  the  chemical  reaction. 
T=  the  absolute  temperature. 

—  —  =  the  temperature  coefficient  of  available  elec- 

trical energy. 

Since  all  our  calculations  are  based  on  gram-equiv- 
alents, 96,540  coulombs  are  always  supposed  to  pass 
through  the  cell,  and  the  electromotive  force  of  the 
cell  is  therefore  a  measure  of  the  available  electrical 
energy. 

If  e  =  the  electromotive  force  of  the  cell,  we  can 
put  this  formula  into  a  form  adapted  specially  for 
the  case  of  galvanic  cells. 

$.mSt+r&> 

F^      dT" 
F  being  our  96,540  coulombs. 

For  an  acid  concentration  corresponding  to  a  den- 
sity of  1.210  we  have  for  Q  (per  gram-equivalent) 
about  43,000  calories. 

—  ^  is  positive  and  has  a  value  of  about  0.0003  at 
20°  C. 


ENERGY  RELATIONS  71 

Numerically 

e  =  43,000x4^  +  [290x0.0008], 


e  =  1.86  +  0.087  =  1.95, 

which  is  a  little  lower  than  the  usual  measurement 
of  2.04  to  2.06  volts. 

The  complete  derivation  of  the  formula  will  be 
found  in  the  Appendix,  page  255. 

This  is  the  general  form  of  the  expression  for  the 
electromotive  fprce  of  a  galvanic  cell  in  terms  of  the 
chemical  heat  of  reaction  and  the  temperature  coeffi- 
cient of  the  electromotive  force.  It  is  perfectly 
general  and  suggests  many  interesting  things.  There 
are  cells  which  warm  up  a  good  deal  while  they  work. 
These  are  the  ones  whose  electromotive  force  de- 
creases rapidly  when  their  temperature  is  raised. 
Others  cool  down,  and  the  reverse  effect  is  produced 
on  these  by  warming  them  from  without.  In  the  first 
class,  part  of  the  energy  of  the  chemical  reaction  is 
used  to  heat  the  room.  In  the  second  class  some 
energy  is  taken  from  the  room  in  the  form  of  heat 
and  converted  in  the  cell  into  electrical  energy. 
There  are  cells  in  which  the  heat  of  the  chemical 
reaction  is  zero  and  in  which  all  the  electrical  energy 
is  produced  at  the  expense  of  heat  absorbed  from 
the  surrounding  air.  These  are  the  "concentration 
cells,"  and  they  are  very  interesting  and  important 


72 


STORAGE  BATTERIES 


theoretically,  even  though  none  of  them  are  used  as 
practical  sources  of  current. 

45.  Temperature  Coefficient.  —  The  usual  commercial 
storage  cell  has  a  fairly  large  positive  temperature 
coefficient  —  about  0.0003  per  Centigrade  degree. 
But  it  gains  no  energy  from  this  fact  because  we 


<X50 


i.l  1.2  1.3  1.4 

DENSITY  OF  ELECTROLYTE! 

FIG.  22. —  Change  in  the  temperature  coefficient  of  the  e.  m.  f.  of  a 
storage  cell  with  change  in  acid  concentration  (density). 

reverse  it  when  we  charge  it  and  lose  from  the 
negative  coefficient  during  this  part  of  the  cycle. 
As  far  as  this  one  factor  is  concerned  we  should 
charge  it  as  cold  as  possible  and  discharge  it  as  hot  as 
possible.  But  as  we  shall  see  later,  temperature  has 
much  larger  and  more  important  influence  on  other 
factors,  and  in  comparison  with  them  the  change  in 


ENERGY  RELATIONS  73 

electromotive  force  with  temperature  is  quite  negli- 
gible. Figure  22  shows  the  change  in  the  electro- 
motive force  of  the  cell  with  change  in  acid  concen- 
tration, and  the  — ^  of  the  formula  can  be  taken 

from  this  curve.  At  acid  concentrations  higher  than 
2  gm.-mol.  per  liter  the  curve  does  not  fit  the  meas- 
urements perfectly,  and  the  values  obtained  by  cal- 
culating backward  from  the  heats  of  dilution  are 
probably  correct.  The  departure  is  not  great,  but 
requires  explanation.  It  may  be  that  the  more  con- 
centrated acid  attacks  and  combines  with  the  lead 
sponge  of  the  negative  plate,  even  when  no  current 
is  passing,  giving  out  heat,  and  this  loss  of  energy 
would  of  course  mean  that  the  electromotive  force 
found  by  measurement  will  be  too  small. 

46.  The  Heat  of  Dilution  of  Sulphuric  Acid.  —  The 
determination  of  the  heat  of  reaction  for  the  mate- 
rials of  the  storage  cell  was  made  in  very  dilute  sul- 
phuric acid.  Under  these  conditions  there  would  be 
set  free  in  the  calorimeter,  besides  the  heat  of  the 
substances  indicated  in  the  equation  for  the  cell  re- 
action, the  heat  of  dilution  of  2  gm.-mols.  of  H2SO4. 
This  is  a  considerable  amount  of  heat,  as  every  one 
knows  who  has  had  occasion  to  dilute  sulphuric  acid 
by  pouring  the  concentrated  acid  into  water.  If  we 
used  very  dilute  acid  in  our  cells,  we  could  also  use 
the  heat  of  reaction  found  in  the  calorimeter,  but 


74 


STORAGE  BATTERIES 


since  we  use  in  practice  rather  concentrated  acid, 
we  evidently  cannot  expect  to  get  any  more  energy 
than  could  be  obtained  from  the  heat  of  the  cell 
materials  plus  the  heat  of  dilution  from  pure  H2SO4 
to  the  acid  concentration  used  in  our  cell. 

The  curves  of  Figures  23  and  24  show  the  heat  of 


1 

§. 


\ 


60         70 


FIG.  23. —  Curve  showing  the  heat  of  dilution  of  a  gram  molecule  of 
H2SO4  to  various  concentrations.  Heat  given  in  thousands  of  calories. 

dilution  of  sulphuric  acid.  Along  the  bottom  of  the 
diagram  of  Figure  24  are  given  the  densities  of  the 
solutions  formed,  and  along  the  top  the  concentra- 
tion of  these  solutions  in  gram-molecules  of  H2SO4 
per  liter  of  solution. 

The  Q  which  we  use  in  our  energy  formula  con- 
sists evidently  of  two  parts,  one  being  the  heat  of 


ENEEGT  RELATIONS 


75 


reaction  of  the  materials  according  to  the  funda- 
mental cell  reaction,  the  other  the  heat  of  dilution 
to  the  concentration  used  in  the  cell  being  tested. 
Since  the  temperature  coefficient  also  plays  a  con- 
siderable part  in  our  calculations  of  electromotive 


HEAT  OF  DILUTION  IN  CALORIES 

i  i  I  i  1  I  I  I  1 

GM.-MOLS.    PER  LITER 

i  —  r 

--v^ 

T—  r 

-T-  T 

1     ' 

1     ' 

1     ' 

1    ' 

1 

N 

N^ 

\ 

X 

s 

\ 

\ 

\ 

\ 

DENSITY 

FIG.  24.  —  Curve  showing  the  heat  of  dilution  of  H2SO4  to  electrolyte 
of  various  densities  (at  bottom)  and  to  various  concentrations  in 
gm.-mols.  per  liter  (at  top). 

force,  the  easiest  way  of  approaching  the  subject 
seems  to  be  to  choose  as  our  starting  point  an  acid 
concentration  such  that  the  cell  has  no  temperature 
coefficient  of  electromotive  force.  This  we  did  by 
choosing  acid  of  density  1.044  (0.70  gm.-mol.  per 
liter),  and  we  thus  made  one  factor  constant. 


76  STORAGE  BATTERIES 

47.  Very  Dilute  Electrolyte.  —  87,000  calories  is  the 
total  heat  of  reaction  when  acid  of  this  density  is 
used  in  the  cell,  and  this  is  already  so  dilute  an  acid 
that  not  very  much  more  heat  could  be  obtained  by 
diluting  it  a  great  deal  further.  It  will  be  seen 
from  the  curve  (Figure  24)  that  the  difference  in 
the  heats  of  dilution  of  0.70  normal  acid  and  0.0 
normal  acid  is  small.  It  is  only  a  couple  of  hun- 
dred calories  at  the  most.  Q  will  therefore  be  about 
87,200  calories  for  the  most  dilute  solution  in  which 
the  cell  electromotive  force  could  be  measured. 

From  Figure  22  we  see  that  the  temperature  co- 
efficient for  very  dilute  acid  is  negative,  and  that  it 
is  rapidly  increasing  in  the  negative  direction  as  the 
acid  density  approaches  zero.  Dolazalek  has  meas- 
ured this  coefficient  for  very  dilute  acid  (0.0005  gm.- 
mol.  per  liter),  and  he  finds  it  about  —  0.0025  volts 
per  Centigrade  degree. 

From  these  data  we  can  calculate  the  electromotive 
force  of  a  storage  cell  having  this  very  dilute  acid  as 
electrolyte. 


dT' 


1.87-0.72  =  1.15  volts, 
which  is  close  to  the  measured  value. 


ENERGY  RELATIONS 


77 


48.  Concentrated  Acid.  —  Passing  to  concentrated 
acid,  the  agreement  between  the  simple  theory  and 
the  measurements  is  not  by  any  means  so  close. 
This  will  be  at  once  evident  from  an  examination 
of  the  curves  of  Figures  22  and  24  in  connection  with 


13 


1.6 


T-i 


GM.-MOLS.  PER   LITER 


PERCENT  OF  H2SO4 

FIG.  25. — Variation  of  cell  voltage  with  change  in  acid  concentration 
(in  percent  of  H2SO4  at  botton,  in  gm.-mols.  per  liter  at  top). 

the  results  of  measurement  on  cells  with  various  acid 
concentration,  given  in  the  curve  of  Figure  25. 
Measurement  shows  that  the  electromotive  force  of 
a  cell  is  nearly  a  linear  function  of  the  acid  concen- 
tration, only  departing  from  a  straight  line  in  the 
region  of  dilute  acid,  and  certainly  approximately 
straight  for  all  acid  concentrations  used  in  practice. 


78  STORAGE  BATTERIES 

Figure  24  shows  that  Q  decreases  with  increasing 
acid  concentration,  since  the  heat  of  dilution  to  be 
subtracted  from  the  constant  part  of  Q  becomes 
greater  and  greater  as  the  acid  concentration  in- 
creases. On  the  other  hand,  the  change  in  electro- 
motive force  with  change  of  temperature  is  in  the 
right  direction  to  counterbalance  this  only  as  far  as 
acid  of  density  1.15.  Beyond  this  both  the  Q  and 

the  —  ^=  of  our  energy  formula  are  decreasing,  while 
cl  J. 

the  measurements  show  that  the  electromotive  force 
is  constantly  increasing. 

At  acid  density  1.15  the  formula  still  holds  accu- 
rately enough. 


e  =  1.85  +  0.14  =  1.99. 

For  the  higher  densities  we  can  no  longer  expect 
close  agreement,  if  we  take  the  data  of  our  curves. 
But  at  the  usual  acid  density  of  1.210  the  agreement 
is  still  fairly  close. 

=  85,000  x  4.18      29Q      aoo032 

2  x  96,540 
e  =  1.84  +  0.093  =  1.933, 

noticeably  lower  than  the  measured  value,  which  is 
2.06  volts. 

49.    This  lack  of  agreement  of  course  arouses  sus- 


ENERGY  RELATIONS  79 

picion  of  our  data.  The  fundamental  theory  has 
been  so  well  and  thoroughly  proven  in  hundreds  of 
cases  that  we  need  hardly  fear  any  trouble  there. 

While  the  thermochemical  data  for  the  heat  of 
reaction  and  the  heat  of  dilution  are  hard  to  obtain 
and  undoubtedly  fraught  with  considerable  experi- 
mental error,  there  is  nothing  in  the  course  of  the 
curves  expressing  them  to  excite  any  suspicion  of 
the  correctness  of  their  general  trend. 

The  curve  connecting  the  temperature  coefficient 
of  the  electromotive  force  with  the  acid  density 
(Figure  22)  is  the  one  which  seems  to  contain  the 

de 
doubtful  data.     The  droop  in  the  value  of  — —  comes 

Ci  JL 

in  those  concentrations  of  acid  where  lead  is  rather 
rapidly  attacked  and  dissolved.  Manufacturers  have 
stopped  increasing  the  density  of  their  electrolyte 
at  about  1.200,  because  they  found  local  action  to 
be  a  factor  just  beyond  that  point.  If  there  is  local 
action  at  the  negative  plate,  and  the  acid  is  being 
used  up  there  as  a  result,  the  average  density  in  the 
cell  would  not  be  the  same  as  that  at  the  point  of 
cell  activity.  And  since  there  is  no  current  passing 
when  these  measurements  are  made,  diffusion  alone 
must  replace  the  exhausted  acid.  This  would  cer- 
tainly account  for  at  least  a  part  of  the  discrepancy, 
but  this  still  remains  a  point  which  demands  further 
investigation. 


CHAPTER   VIII 
REACTIONS  AT  THE  ELECTRODES 

50.  In  our  discussion  of  the  action  of  the  Daniell 
cell  (page  26)  we  decided  that  we  could  get 

1.1  x  96,540  volt-coulombs 

of  work  from  the  cell  when  32.7  gm.  of  zinc  went  into 
solution  as  Zn++  and  31.8  gm.  of  copper  changed  from 
Cu++  to  metal.  There  are  a  great  number  of  pos- 
sible cells  of  the  same  type,  for  we  can  replace  either 
zinc  or  copper  or  both  by  any  other  metals  immersed 
in  solutions  of  their  salts,  and  in  this  way  make  cells 
quite  similar  to  the  prototype. 

51.  Cells  of  the  Daniell  Type. — The  following  list 
indicates  a  few  of  the  combinations  and  their  electro- 
motive forces.     These  are  measured  with  the  metal 
immersed  in  a  solution  which  is  normal  with  respect 
to  the  metallic  ion.     The  Daniell  cell  itself  contains 
65.4  gm.  of  Zn++  per  liter  of  solution  about  the  anode 
and  63.6  gm.  of  Cu++  per  liter  about  the  cathode. 
Whenever  we  use  silver  as  electrode,  we  measure  it 
in  a  silver  salt  solution  containing  107.9  gm.  of  Ag+ 
per  liter. 

80 


REACTIONS  AT  THE  ELECTRODES  81 

e.  m.  f. 

Cu/Cu+yZn++/Zn  I-10  Cu  cathode,  Zn  anode 

Cu/Cu++/Cd+VCd  °-750  Cu  cathode,  Cd  anode 

Cu/Cu++/Fe+yFe  0.986  Cu  cathode,  Fe  anode 

Cu/Cu++/Ni+YNi  0.926  Cu  cathode,  Ni  anode 

Cu/Cu++/Ag+/Ag  °-469  Ag  cathode,  Cu  anode 

Zn/Zn++/Cd+Vcd  °-350  Cd  cathode,  Zn  anode 

Zn/Zn++/Fe+VFe  °-113  Fe  cathode,  Zn  anode 

Zn/Zn++/Ni++/Ni  0.173  Ni  cathode,  Zn  anode 

Zn/Zn+YAg+/Ag  !-568  Ag  cathode,  Zn  anode 

etc. 

If  a  very  little  cross-calculation  is  undertaken, 
some  interesting  things  will  be  found.  We  did  not 
need  nearly  all  these  statements  to  cover  the  facts, 
for  we  can  calculate  from 

Cu/Cu+VZn++/Zn  =  1.10 
Cu/Cu++/Cd+yCd  =  0.750 
Cd/Cd++/zn++/Zn  =  0.350 

and  others  in  the  same  way.  We  can  also  calculate 
a  good  many  combinations  which  we  have  not  put 
down.  For  example,  — 

Zn/Zn++/Ni++/Ni  =  0.173 
Zn/Zn++/Ag+/Ag  =  1.568 
Ni/Ni++/Ag+/Ag  =  1-395 
and  in  the  same  way  for  any  other  combination. 

All  these  connected  facts  suggest  a  possible  sim- 
plification. Why  not  calculate  the  work  at  the  two 


82  STORAGE  BATTERIES 

electrodes  separately  ?  For  the  Daniell  cell :  (1)  the 
work  available  when  31.8  gm.  of  copper  changes 
from  ion  to  metal,  and  (2)  the  work  available  when 
32.7  gm.  of  zinc  changes  from  metal  to  ion.  And 
of  course  we  would  not  stop  here.  We  would  go  on 
and  determine  the  work  available  when  107.9  gm. 
of  silver  passed  a  silver  electrode,  and  so  on  for  all 
the  single  electrodes.  Dividing  the  work  in  joules 
in  each  case  by  96,540,  we  would  then  have  a  series 
of  single  electromotive  forces,  and  from  this  series 
we  could  pick  out  any  two  we  wished  to  combine  to 
make  a  galvanic  celL 

52.  Standard  Electrode.  —  Before  we  can  begin  to 
make  such  a  series  we  must  in  some  way  fix  a  value 
for  one  single  electromotive  force  metal/ion0  There 
has  been  a  good  deal  of  trouble  in  scientific  circles 
about  this,  but  fortunately  it  does  not  make  the  least 
difference  for  our  elementary  work  what  this  stand- 
ard metal/ion  electrode  is,  or  what  we  take  for  its 
single  electromotive  force.  If  we  should  put  any 
one  of  the  single  metal/ion  combinations  equal  to  1, 
and  then  measure  all  the  others  against  this,  we 
would  arrive  at  exactly  the  same  figures  as  those 
given  in  our  series  on  page  81.  As  a  matter  of  fact 
we  have  a  so-called  "normal  electrode,"  and  its  elec- 
tromotive force  has  been  determined  separately 
in  various  ways.  Measured  against  this  single 
electrode,  it  has  been  found  that  the  electromotive 


REACTIONS  AT  THE  ELECTRODES  83 

force  Zn/Zn"1"4"  has  the  value  1.053  volts,  the  zinc 
passing  from  metal  to  ion  through  the  electrode.  It 
is  given  the  negative  sign  and  is  written  Zn/Zn++ 
=  -1.053. 

Cu/Cu++  is  +0.046,  measured  against  the  same 
standard. 

Using  these  values,  and  our  series  of  cells  of  the 
Daniell  type,  it  is  a  very  easy  matter  to  write  out  a 
list  of  the  single  potentials  of  all  the  metal/ion 
electrodes  which  appear  in  that  list. 


Fe/Fe++  -0.940 

Ni/Ni++  -0.880 

Cd/Cd++  -  0.703 

Ag/Ag+  +  0.505 

and  we  might  add  from  other  measurements 

Pb/Pb++        -  0.431 
H/H+  -0.283 

Hg/Hg++      -  0.46T,  etc. 

53.  Work  done  at  an  Electrode.  —  So  here  we  have 
the  way  opened  for  the  calculation  of  the  work  done 
at  each  electrode.  We  need  only  to  multiply  the 
single  electromotive  force  by  96,540  and  the  result 
is  the  number  of  joules  furnished  by  that  half  of  the 
cell  during  the  change  of  a  gram-equivalent  of  the 
metal  to  ion,  or  vice  versa.  There  would  not  be  much 


84  STORAGE  BATTERIES 

need  for  any  more  minute  theory  of  the  process  if 
the  single  electrodes  did  not  change  their  electro- 
motive force  considerably  when  the  ion  concentration 
about  them  is  changed.  For  instance,  if  we  are 
using  Ag/Ag+  as  one  of  our  electrodes  and  silver  is 
going  out  of  solution,  this  half  of  the  cell  furnishes 
0.515  x  96,540  joules  of  work.  But  if  we  change 
the  concentration  of  the  ion  from  107.9  gm.  per  liter 

to  10.79  gm.  per  liter  (from  N  to  —  \  the  half  cell 

only  furnishes  0.457  x  96,540  joules  for  the  same 
amount  of  silver. 

At    the   anode    a   change    of    concentration    has 
the   opposite   effect.      Zn/Zn++  N  has   1.053   volts. 

+       measures  1.082  volts. 


Nernst  has  suggested  a  generalization  which  makes 
the  whole  subject  matter  easy  to  remember  and 
which  at  the  same  time  opens  the  way  to  many  inter- 
esting and  important  numerical  relations. 

54.  Nernst's  Theory  of  Solution  Pressure.  —  Let  us 
think  of  the  question  in  this  way:  Each  metal  has 
a  tendency  to  send  ions  into  solution,  and  does  it. 
The  ions  carry  with  them  a  definite  quantity  of 
electricity  of  +  sign,  for  the  metallic  ions  are  all 
cations.  If  the  electric  circuit  is  not  a  closed  one, 
this  leaves  the  metal  with  —  charge,  and  before  the 
concentration  of  ions  has  reached  a  very  high  value, 


BE  ACTIONS  AT  THE  ELECTRODES  85 

a  true  static  attraction  is  produced  between  the  — 
charged  plate  and  the  +  charged  ions  in  solution. 
Unless  this  condition  of  things  is  relieved  by  dis- 
charging the  plate,  the  concentration  of  the  ion  in  so- 
lution no  longer  increases,  and  we  have  equilibrium. 
(See  Fig.  21.) 

Theoretically,  at  least,  we  can  reverse  this  process 
by  using  a  metal  with  a  comparatively  slight  tend- 
ency to  go  into  solution,  and  placing  it  in  a  con- 
centrated solution  of  its  ion.  Since  a  very  small 
concentration  of  ion  is  necessary  to  balance  the 
solution  pressure  of  the  metal  and  we  have  purposely 
made  the  ionic  concentration  high,  ion  will  change 
to  metal  under  these  circumstances  and  the  plate 
will  take  on  a  +  charge  until  static  repulsion  causes 
equilibrium.  So  far  this  is  rather  hypothetical.  But 
measurements  show  that  it  fits  the  facts  very  closely 
indeed.  If  a  metal  is  going  into  solution  as  part  of 
a  galvanic  arrangement,  we  can  better  the  electromo- 
tive force  of  the  cell  by  surrounding  this  anode  with 
an  ionic  concentration  as  small  as  possible.  The 
single  electromotive  force  of  the  electrode  goes  up 
as  the  solution  about  it  is  diluted.  If  a  metal  is  to 
go  out  of  solution  as  part  of  a  cell,  we  can  assist  it 
by  increasing  the  concentration  of  its  ion  to  as  high 
a  value  as  possible. 

55.  Electrode  Equilibrium.  —  A  few  simplifying 
assumptions  lead  us  to  still  more  exact  numerical 


86  STORAGE  BATTERIES 

relations.  Let  us  assume  that  the  solution  pressure 
of  each  metal  is  constant  and  that  when  it  dips  in  a 
solution  it  is  constantly  held  in  equilibrium  by  a 
layer  of  charged  ions  about  it.  Then  the  passage 
of  96,540  coulombs  through  the  cell  results  in  the 
change  (suppose  this  is  the  anode)  of  a  gram-equiva- 
lent of  metal  into  ions  of  this  definite  equilibrium 
concentration  and  subsequent  diffusion  of  these  ions 
from  the  more  concentrated  solution  about  the  plate 
into  the  main  body  of  the  electrolyte.  The  whole 
work  of  the  electrode  has  been  expended  in  main- 
taining this  ion  concentration  about  the  plate.  We 
can  calculate  the  total  work  of  the  electrode  as  merely 
the  osmotic  work  corresponding  to  the  change  of  a 
gram-equivalent  of  the  ion  from  its  equilibrium  con- 
centration to  the  average  concentration  of  the  elec- 
trolyte (see  Appendix,  page  256). 

56.   Osmotic  Work.  —  The  osmotic  work  available 
as  the  result  of  such  a  change  in  concentration  is 


where  O1  is  the  concentration  in  the  equilibrium 
layer  about  the  electrode,  <72  the  concentration  in  the 
main  body  of  the  cell,  R  is  a  constant  for  all  dilute 
solutions  —  numerically  the  same  as  the  gas  constant 
R,  T  is  the  absolute  temperature,  and  In  is  the  sign 
indicating  a  logarithm  to  the  natural  base  e. 


REACTIONS  AT  THE  ELECTRODES  87 

01  was  the  concentration  which  exactly  balanced 
the  solution  pressure  of  the  metal.  As  far  as  we 
are  concerned  we  could  put  P,  the  solution  pressure 
of  the  metal,  in  place  of  C^  since  the  electrode  is 
in  equilibrium. 

Now  let  a  gram-equivalent  of  the  metal  change  to 
ion  and  diffuse  into  a  very  large  cell,  in  which  the 
ionic  concentration  is  <72. 

The  osmotic  work  is 


and  since  a  gram-equivalent  has  been  used,  96,540 
coulombs  have  passed  through  our  electric  circuit. 
Electromotive  force  x  96,540  =  osmotic  work 


The  electrode  electromotive  force 

RT 


96,540      <72 

If  we  put  in  the  numerical  values,  using  the  gas 
constant  for  R  and  changing  it  to  joules,  measuring 
everything  at  17°  C.,  and  changing  to  the  ordinary 
system  of  logarithms,  we  get 


n  being  the  valence  of  the  ion. 


88  STORAGE  BATTERIES 

This  for  one  electrode.  At  the  other  we  will  have 
a  precisely  similar  set  of  relations  except  that  at  the 
cathode  the  change  is  from  ion  to  metal,  and  the 
electromotive  force  will  therefore  have  the  opposite 
sign.  The  electromotive  force  of  the  cell  as  a  whole 
will  be  the  difference  of  the  two  expressions. 


57.  Effect  of  Concentration  on  Electromotive  Force.  — 
Evidently  if  we  want  our  cell  to  have  a  high  electro- 
motive force,  we  must  choose 

as  anode,  a  metal  with  a  high  solution  pressure  ; 
as  cathode,  a  metal  with  a  low  solution  pressure. 
And  we  must  also  make 

the  ion  concentration  about  the  anode  low  ; 
the  ion  concentration  about  the  cathode  high. 

58.  Application  to  Lead  Accumulator.  —  In  the  case 
of  the  lead  accumulator  we  have  evidently  chosen 
a  favorable  set  of  conditions,  for  it  has  about  as  high 
an  electromotive  force  as  any  practicable  cell.     It  is 
a  matter  of  interest  to1  examine  this  particular  gal- 
vanic combination  from  the  new  point  of  view. 

No  difficulty  is  found  in  applying  it  to  the  lead 
plate.  This  is  the  anode  during  discharge,  and  we 
can  be  quite  sure  that  this  electrode  is  reversible 
with  respect  to  the  ion  Pb++.  We  have  insured  a 
low  concentration  of  this  ion  in  the  main  body  of  the 


REACTIONS  AT  THE  ELECTRODES  89 

electrolyte,  for  lead  sulphate  is  a  very  slightly  solu- 
ble substance.  The  only  electrolyte  which  I  can 
think  of  that  would  possibly  increase  this  single 
electromotive  force  would  be  a  soluble  sulphide,  for 
lead  sulphide  is  even  less  soluble  than  the  sulphate. 
For  the  lead  plate,  we  have 

e  =  0.0288  log 

59.  Theory  of  Le  Blanc.  —  When  we  examine  the 
peroxide  plate  we  find  it  a  much  more  difficult 
matter  to  decide  upon  our  active  ion.  Whatever  it 
is,  it  must  be  present  in  the  electrolyte  in  exceedingly 
small  concentration  and  quite  beyond  the  limits  of 
chemical  analysis.  Two  theories  have  been  pro- 
posed, one  by  Le  Blanc  and  one  by  Liebenow,  and 
while  each  assumes  the  existence  and  importance  of 
a  quite  different  ion,  the  final  result  is  much  the 
same  in  each.  Le  Blanc's  reasoning  is  in  this  form. 
Lead  peroxide  has  a  small  but  perfectly  definite 
solubility  in  water,  and  reacts  with  it  in  the  reaction 

Pb02  +  2  H20  =  Pb++  +  4  OH-, 

++ 
forming  a  quadrivalent  lead  ion  Pb++,  and  OH~  ion. 

During  discharge  the  quadrivalent  lead  ion  changes 
to  ordinary  lead  ion  Pb++,  and  this  meets  with  SO4 — 
and  is  precipitated  as  solid  lead  sulphate. 


90  STORAGE  BATTERIES 

The  entire  course  of  discharge  is  therefore  given 
by  the  set  of  equations  — 


Pb02  +  2  H20  =  Ph      4  OH-. 

PbK  +  Pbmet  +  2  S04-  =  2  PbS04, 
4  OH-  -f-  4  H+  =  4  H2O, 

and  during  charge   these   reactions   are   completely 
reversed  :  — 


2  Pb++  =  Pb++  +  Pbmet, 

Pb++  +  4  OH-  =  PbO2  +  2  H20, 
4  H+  +  2  SO4~  =  2  H2SO4. 

The  total  result  of  these  reactions  gives  a  reaction 
just  like  our  fundamental  one  — 

Pb  +  Pb02  +  2  H2S04  =  2  PbS04  +  2  H20, 

for  during  discharge  we  lose  lead  and  lead  peroxide 
and  gain  2  of  lead  sulphate  and  2  of  water,  and  dur- 
ing charge  the  reverse  change  takes  place.  As  far 
as  the  chemical  facts  of  the  reaction  are  concerned, 

Le  Blanc's  theory  fits  very  well. 

++ 
The  quadrivalent  lead  ion  Pb++  can  be  shown  to 

exist,  but  we  have  not  much  data  as  to  its  concentra- 
tion in  the  electrolyte  of  a  lead  accumulator. 

60.  Liebenow's  Theory.  —  Liebenow's   theory   is   in 
several  ways  a  more  acceptable  one  than  Le  Blanc's. 


REACTIONS  AT  THE  ELECTRODES  91 

He  assumes  that  the  lead  peroxide  electrode  is  re- 
versible and  that  the  electrolyte  contains  PbO2  ion. 
Then  during  discharge  this  ion  goes  into  solution  at 
the  cathode  (it  is  a  negative  ion)  and  reacts  with  the 
H+  ion  of  the  acid  to  form  Pb  and  water 

PbO2--  +  4  H+  =  Pb++  +  2  H2O; 
the  lead  ion  finds  SO4      ion  waiting  for  it, 
Pb++ +  S04— =  PbS04, 

and  precipitates  as  solid  lead  sulphate  (see  Figures 
14  and  15). 

The  reaction  at  the  anode  is  the  same  as  before, 
and  the  sum  of  the  whole  is  again  our  fundamental 
reaction. 

PbO2  undoubtedly  does  exist  in  perfectly  meas- 
urable concentration  in  strongly  alkaline  solution, 
and  theoretically  must  also  be  present  in  the  acid  of 
the  cell.  In  the  Appendix  (page  261)  will  be  found 
the  complete  calculation,  which  leads  to  the  remark- 
able result  that  the  concentration  of  PbO2 —  in  an 
ordinary  cell  acid  is  about  4  x  10~50  gm.-mols.  per 
liter.  In  the  same  electrolyte  the  concentration  of 
the  Pb++  ion  is  about  2  x  lO'8. 

While  it  is  true  that  10"50  means  only  a  few  mole- 
cules in  a  volume  equal  to  the  oceans  of  the  world, 
this  is  the  number  we  need  to  express  the  concentra- 
tion ratio  in  our  cell.  It  must  be  remembered  that 


92  STORAGE  BATTERIES 

these  ions  only  have  to  pass  over  molecular  distances 
and  that  the  reservoir  of  sulphate  from  which  they 
are  drawn  can  supply  them  as  fast  as  they  are  needed. 
In  such  statistical  matters  as  this  the  unit  may  make 
a  great  difference.  There  is  nothing  surprising 
about  the  statement  that  ten  children  are  born  per 
year  in  a  certain  village.  The  same  fact  is  repre- 
sented by  the  statement  that  0.00000031  children  are 
born  there  per  second. 

In    terms    of    Nernst's    theory    and    Liebenow's 
hypothesis,  we  have  for  the  lead  peroxide  electrode 

«rbo,=  -0.0288  log 
and  for  the  entire  cell 

e  =  0.0288  log     •Pp 


61.  Conclusions  to  be  Drawn.  —  This  equation  gives 
interesting  qualitative  relations.  Evidently  we  can 
hardly  do  better  than  to  retain  sulphuric  acid  as  our 
electrolyte.  We  are  also  to  use  it  as  strong  as  the 
life  of  the  plates  will  permit  ;  for  while  lead  sulphate 
is  more  soluble  in  concentrated  acid  than  in  dilute, 
and  we  will  therefore  lose  a  little  at  the  lead  elec- 
trode, the  PbO2  concentration  decreases  as  the 
fourth  power  of  the  hydrogen  ion  concentration,  and 
we  should  much  more  than  make  up  for  the  loss. 
As  a  matter  of  fact,  manufacturers  have  gradually 


REACTIONS  AT  THE  ELECTRODES  93 

increased  the  commercial  concentration  of  their  elec- 
trolyte, with  a  corresponding  increase  in  the  electro- 
motive force  of  their  cells.  Ten  years  ago  electrolyte 
of  density  1.15  was  the  rule.  Now  nearly  every  one 
uses  a  density  of  1.210,  and  for  special  work  as  high 
as  1.225.  In  portable  cells  where  the  limit  of  weight 
is  fixed  and  a  small  total  mass  of  electrolyte  must  be 
carried,  the  density  is  permitted  to  go  as  high  as  1.27. 
We  can  also  see  from  this  formula  that  an  alkaline 
electrolyte,  with  its  high  concentration  of  PbQ2  , 
would  greatly  decrease  the  electromotive  force  of 
the  cell.  In  caustic  soda  solution  it  does  in  fact  go 
as  low  as  0.75  volt.  An  electrolyte  containing  a 
large  concentration  of  Pb++  will  also  lower  the  elec- 
tromotive force,  and  if  we  could  manage  an  electro- 
lyte which  was  both  strongly  alkaline  and  high  in 
Pb++,  we  could  reach  a  very  low  value  indeed. 


CHAPTER  IX 

CHARGE  AND  DISCHARGE 

62.  Up  to  now  we  have  been  considering  the  cell 
as  independent  of  the  current  flowing  through  it. 
This  point  of  view  is  necessary  for  a  theoretical  dis- 
cussion, because  the  whole  cell  is  changed  as  soon  as 
current  passes.  From  a  rather  simple  system,  quite 
open  to  formal  investigation  as  long  as  it  stands  on 
open  circuit,  the  cell  changes  to  a  very  complex 
system  as  soon  as  it  begins  to  work.  The  only  way 
to  study  this  complicated  thing  is  to  keep  all  the 
factors  but  one  as  constant  as  possible,  and  follow  the 
change  in  that  one.  Each  factor  in  turn  can  some- 
times be  taken  up  in  this  way  and  the  whole  problem 
cleared  up.  But  in  the  case  of  our  cell  we  shall  find 
that  this  general  method  of  solving  scientific  puzzles 
is  hard  to  apply.  So  many  of  the  factors  which  are 
active  in  a  storage  cell  are  not  within  our  direct  con- 
trol. For  these  reasons  it  is  easiest  to  follow  the 
changes  in  an  accumulator  by  study  of  curves  and 
families  of  curves.  A  single  such  curve  shows  the 
mutual  effect  of  two  things.  A  family  of  curves 
shows  a  great  deal  about  three  factors  and  their  re- 

94 


CHARGE  AND  DISCHARGE  95 

lations.  Let  us  take  first  of  all  the  curves  which 
show  how  the  voltage  of  an  accumulator  changes 
with  time,  while  it  is  being  charged  and  discharged 
at  a  constant  rate. 

In  all  that  follows,  the  general  theory  of  Chap- 
ter VIII  should  be  kept  clearly  in  mind.  Large 
changes  in  voltage  appear  during  complete  charge 
and  discharge,  but  every  change  can  be  explained 
satisfactorily  and  completely  by  reference  to  changes 
in  the  concentration  of  the  active  ions. 

The  electromotive  force  of  the  Pb/Pb++  electrode 
is  given  by  the  formula  — 

e=  0.0288  lo-^a,. 


and  that  of  the  PbO2/PbO2~  electrode  by  — 
efbo,  =  0.0288  log 

at  every  point  of  a  charge,  discharge,  or  recovery 
curve. 

The  only  variables  are  the  concentrations  of  Pb++ 
and  PbO2—  . 

It  should  also  be  kept  clearly  in  mind  that  the 
Pb++  ion  concentration  varies  inversely  as  the  acid 
concentration  at  the  point  of  activity,  and  inversely 
as  the  square  of  the  H+  ion  concentration,  while  the 
PbO2  —  ion  concentration  varies  inversely  as  the 


96  STORAGE  BATTERIES 

square  of  the  acid  concentration  and  therefore  in- 
versely as  the  fourth  power  of  the  H+  concentration 
(see  Appendix,  page  260,  for  the  complete  state- 
ment of  the  theory). 

63.  Charge  Curve.  —  Our  cell  has  been  fully  dis- 
charged at  a  rather  low  rate.  Lead  sulphate  has 
been  formed  through  each  plate  wherever  sulphuric 
acid  of  sufficient  concentration  was  available  for  re- 
action. Lead  peroxide  and  lead  sponge  have  been 
more  or  less  completely  exhausted  and  partially 
covered  with  a  layer  of  sulphate.  Sulphuric  acid 
has  been  taken  from  the  electrolyte,  which  has  a 
lower  acid  concentration  than  before  the  discharge. 

We  connect  -the  terminals  of  the  cell  with  a  source 
of  current,  and  proceed  to  charge  it. 

The  reaction  is 

2  PbSO4  +  2  H2O  =  PbO2  +  Pb  +  2  H2SO4. 

The  reservoir  of  lead  sulphate  supplies  material, 
and  water  is  taken  from  the  electrolyte  as  well. 
The  reactions  described  on  page  55  begin,  and  sul- 
phuric acid  is  set  free  in  the  two  plates. 

If  the  cell  has  been  recently  discharged,  this  reaction 
begins  immediately,  and  the  voltage  rises  slowly 
until  diffusion  balances  the  concentration  of  the  acid 
at  the  point  where  the  reaction  is  taking  place.  But 
if  the  cell  has  been  rather  completely  discharged, 
and  has  been  standing  for  some  time,  the  layer  of 


CHARGE  AND  DISCHARGE 


97 


sulphate,  which  has  had  time  to  change  into  the 
firmer  and  more  stable  modifications,  must  first  be 
broken  through.  In  this  case  the  charging  voltage 
overshoots  a  little  just  at  first  (Figure  26).  It  rises 
rapidly  for  a  short  time,  and  then  drops  again  slowly 
to  the  value  corresponding  to  the  concentration  of 


tx\ 


2.081 


\ 


678 

MINUTES 


FIG.  26. — The  very  beginning  of  charge  on  a  completely  discharged 
plate.     (Vertical  scale  large.) 

the  acid  at  the  active  point  in  the  plate  (see  A, 
Figure  27).  There  is  no  positive  evidence  that  this 
kind  of  lead  suphate  is  an  insulator  or  even  a  very 
poor  conductor.  Measurements  of  the  internal  re- 
sistance of  a  discharged  cell  show  that  there  is  no 
increase  at  this  point  sufficient  to  account  for  this 
little  rise  in  voltage.  It  seems  much  more  probable 


98 


STORAGE  BATTERIES 


that  the  acid  concentration  is,  as  usual,  responsible, 
and  that  the  layer  of  sulphate  merely  prevents  easy 
diffusion  until  it  has  been  broken  through.  It  may 
act  for  the  moment  as  a  semi-  or  nearly  impermeable 
membrane,  retaining  the  concentrated  acid,  and  so 
causing  the  rise  in  electromotive  force. 


ae 


24 


A  B 


2345 

HOURS 

Fia.  27.  —  Changes  in  cell  e.  m.  f.  during  charge  and  discharge  at  the 
5-hour  rate. 

In  any  case  the  electromotive  force  of  our  cell 
very  soon  reaches  a  definite  value,  characterized  by 
the  factors  :  — 

(a)  Acid  density. 

(5)  Temperature. 

(c)  Rate  of  charge. 

(d)  Type  of  plate. 
(e~)  Previous  history. 


CHARGE  AND  DISCHARGE  99 

64.  Peculiarities  of  the  Charge  Curve.  —  At  the  point 
marked  B  on  the  charge  curve  (Figure  27)  this 
definite  condition  has  been  reached.  The  condition 
is  only  momentary,  and,  as  charge  proceeds  at  con- 
stant rate,  the  electromotive  force  increases  slowly 
throughout  the  part  of  the  curve  marked  0.  Sulphate 
is  being  transformed  into  lead  and  peroxide,  and  acid  is 
being  produced  throughout  the  plates.  Diffusion  is 
becoming  more  and  more  difficult,  for  it  must  take 
place  through  ever-increasing  distances,  and  along  tor- 
tuous and  minute  passages.  The  slope  at  any  point  in 
this  part  of  the  curve  is  also  a  function  of  the  five  fac- 
tors, and  the  condition  of  the  cell  as  to  charge  can 
always  be  seen  by  one  acquainted  with  the  type  of 
plate,  by  merely  reading  the  voltmeter,  and  taking 
into  account  the  time  the  cell  has  been  on  charge. 

At  D  there  comes  an  evident  change.  The  curve 
begins  to  rise  much  more  rapidly,  and  gas  is  evolved 
more  freely.  The  curve  rises  through  E,  then  drops 
slightly  at  F,  and  runs  along  parallel  to  the  time 
axis.  From  this  time  on  the  cell  is  merely  a  machine 
for  the  electrolytic  manufacture  of  hydrogen  and 
oxygen. 

The  rapid  change  of  curvature  at  D  is  significant. 
It  cannot  be  due  to  any  further  increase  in  the  acid 
concentration  inside  the  plates,  for  they  are  nearly 
completely  changed  into  lead  and  peroxide  by  now, 
and  very  little  acid  is  being  formed.  What  little  is 


100  STORAGE  BATTERIES 

formed  is  greatly  assisted  in  circulation  and  dilution 
by  the  gas  bubbles  now  rising  from  the  plates.  This 
acts  as  a  vigorous  stirrer  and  equalizes  the  acid  con- 
centration through  the  whole  cell.  The  rapid  rise  at 
D  must  have  another  cause.  Refer  to  the  equation 
on  page  89. 

Up  to  the  point  D  we  had  plenty  of  lead  sulphate 
to  work  on,  and  the  solution  has  always  been 
thoroughly  saturated  with  PbSO4,  except  perhaps 
immediately  about  the  grains  on  which  Pb  and  PbO2 
are  depositing.  But  at  D  we  begin  to  clear  out  the 
last  of  the  solid  sulphate  and  from  that  point  on  the 
solution  becomes  less  and  less  concentrated  in  Pb++. 
Part  way  up  the  curve  at  E  there  is  so  little  Pb++  pres- 
ent that  it  is  just  as  easy  to  cause  hydrogen  gas  to 
leave  the  solution  as  it  is  to  force  out  solid  lead. 
This  means  a  high  electromotive  force  (page  92).  At 
E  the  last  of  the  more  concentrated  acid  and  of  lead 
ion  as  well  hold  up  the  electromotive  force  for  an  in- 
stant by  their  presence  inside  the  plates  ;  they  are 
then  cleared  away  by  streams  of  gas  bubbles,  and  the 
charge  is  complete. 

65.  Now  for  the  factors  a,  5,  0,  d,  and  e,  and  their 
effect  on  the  charge  curve. 

(a)  Acid  density.  The  effect  of  various  concentra- 
tions of  acid  on  the  open  circuit  electromotive  force 
of  the  cell  is  shown  in  Figure  25.  The  effect  at  any 
point  in  the  charge  curve  might  also  be  found,  but 


CHARGE  AND  '! 

it  would  be  so  very  lively  and  changeable  a  factor 
as  not  to  be  very  valuable  as  a  criterion.  From 
what  we  have  already  learned  of  the  effect  of  acid 
concentration  on  electromotive  force  (page  92)  we 
can  be  sure  that  something  like  the  following  picture 
expresses  the  factor  in  question.  Diffusion  is  a  func- 
tion of  gradient.  Acid  will  diffuse  out  of  the  plate 
into  the  ambient  electrolyte  at  a  rate  proportional  to 
the  difference  of  concentration  at  these  two  places. 
But  acid  is  produced  in  the  interior  of  the  plate  in 
direct  proportion  to  the  current  which  is  passing,  and 
regardless  of  acid  density  in  the  electrolyte.  The 
same  current  will  therefore  give  a  greater  gradient 
with  a  weaker  acid  in  the  cell  than  with  a  strong  one, 
and  the  effect  of  the  average  acid  on  the  electromotive 
force  will  be  less  for  high  than  for  low  concentrations. 
(b)  Temperature.  This  has  an  important  effect 
on  diffusion.  At  the  higher  temperature  diffusion 
is  rapid,  and  the  concentrated  acid  formed  in  the 
plate  is  rapidly  removed.  The  voltage  required  to 
charge  our  cell  will  be  lower  and  the  whole  charge 
curve  will  be  changed  in  position  and  shape.  This 
effect  is,  of  course,  quite  aside  from  any  effect  of 
temperature  on  the  electromotive  force  of  the  cell  (see 
page  72),  and  the  latter  factor  is  for  any  practical  cell 
so  small  as  to  be  almost  negligible,  while  the  former 
factor  is  by  no  means  a  small  one.  The  temperature 
coefficient  of  diffusion  is  about  2  %  per  Centigrade  de- 


102  •;  8: 


BATTERIES 


gree  and  is  for  certain  types  of  cell  of  great  importance. 
In  electric  vehicle  work,  for  instance,  winter  tempera- 
tures are  most  trying,  and  the  effect  is  to  reduce  the 
apparent  capacity  of  the  battery  by  a  considerable 
fraction.  This  almost  wholly  because  of  voltage 
limits  imposed  by  the  slowness  of  diffusion  at  the 


2.8 


2.6 


2.2 


2.0 


1.8 


7 


34567 
HOURS 

FIG.  28.  —  Charge  curves  on  the  same  plate  at  various  rates. 

low  temperature.  (See  page  253  for  data  on  practical 
cells.) 

(<?)  Hate  of  charge.  This  determines  the  rate  at 
which  acid  is  formed  at  the  place  where  the  action  is 
going  on.  Diffusion  determines  how  fast  this  acid 
shall  be  removed.  At  high  rates  the  whole  charge 
curve  is  steeper.  (See  Figure  28.) 

(d)  Type  of  plate.  The  position  and  slope  of  the 
charge  curve  vary  with  the  plate  tested.  Surface, 
thickness  of  active  material,  hardness,  are  all  factors. 


CHARGE  AND  DISCHARGE 


103 


A  large-surface  Plante  plate,  with  a  comparatively 
small  content  of  active  material,  shows  a  curve  like 
A  in  Figure  29.  An  intermediate  type  has  the 
characteristics  shown  by  B,  in  the  same  figure.  The 
extreme  of  high  capacity,  a  light  grid  with  a  large 


1- 

-123 


2.0 


12345 
HOURS 

FIG.  29.  —  Charge  curves  for  plates  of  various  types. 
A.  Plants  plates.    B.   Mixed  type.    C.  Paste  plates. 

percentage  of  active  material,  gives  curve  (7;  all  other 
factors  of  course  being  constant  for  the  three  cases. 
Here,  as  in  every  other  case,  the  concentration  of 
acid  at  the  point  of  action  is  the  deciding  factor. 
The  large  surface  plate  is  pretty  freely  open  to  the 
acid.  Diffusion  is  easy,  since  it  takes  place  largely 
through  the  main  body  of  the  electrolyte  and  not 
through  the  pores  of  a  packed  mass  of  active  material. 
In  the  mass  plate  we  have  the  other  extreme. 


104 


STORAGE  BATTERIES 


Diffusion,  except  at  the  very  outside  surfaces,  must 
proceed  through  long  capillaries  in  a  comparatively 
thick  mass  of  active  material  and  is  correspondingly 
slow  and  inefficient. 

66.   Recovery   after    Charge.  —  Our    cell    is    fully 
charged.     The  last  remnants  of  available  lead  sul- 


TIME  Of  CHARGE-MINUTES 

FIG.  30.  —  Curve  showing  end  of  charge  and  recovery  after  opening 

circuit. 

phate  have  been  attacked  and  removed  and  the  plate 
is  nearly  pure  lead  or  lead  peroxide.  Whatever 
sulphate  is  left  in  the  plate  lies  too  deep  to  be  easily 
reached  or  is  incapsulated  with  active  material. 
When  the  charge  circuit  is  broken  the  electromotive 
force  drops  along  a  recovery  curve.  Lead  sulphate 
will  now  go  into  solution  until  saturation  is  reached, 
and  the  process  of  solution  of  the  sulphate  in  the 
quiet  electrolyte  is  largely  one  of  diffusion.  The 


CHARGE  AND  DISCHARGE 


105 


curve  is  very  much  like  a  diffusion  curve,  dropping 
rapidly  at  first  and  then  more  and  more  slowly 
toward  a  limit.  (See  Figure  30.) 

67.   Discharge. —  If  current  be  now  drawn  from  the 
cell  by  closing  the  circuit  through  an  external  resist- 


£06 


u 

O    &6 

$ 

d    2-04 

S 


10        12        14       16        16       20       22. 


MINUTES 

FIG.  31.  —  The  beginning  of  discharge  after  complete  charge.    This 
curve  is  an  enlargement  of  the  first  part  of  the  lower  curve  in 

Figure  27. 

ance,  the  electromotive  force  passes  through  the 
stages  shown  in  the  curve  of  Figure  27.  The  little 
hump  in  the  curve  at  Gr  (see  Fig.  31)  appears  only 
under  certain  conditions,  and  it  may  be  due  to  the 
formation  of  a  supersaturated  Pb++  solution  and  a  cor- 
respondingly low  electromotive  force.  This  could 


106  STORAGE  BATTERIES 

occur  in  very  fully  charged  plates  where  there  is  not 
enough  lead  sulphate  near  the  surface  to  release  such 
a  supersaturatioii.  And,  as  a  matter  of  fact,  it  only 
does  appear  in  fresh  and  active  plates  which  have 
been  very  fully  charged  immediately  previous  to  tak- 
ing the  discharge  curve.  This  peculiar  twist  can  last 
but  an  instant,  for  then  the  limit  of  supersaturation 
is  passed  and  PbSO4  begins  to  deposit  everywhere. 
The  electromotive  force  then  rises  to  its  proper  value, 
corresponding  to  the  concentration  of  the  acid  (now 
being  depleted)  at  the  point  of  activity,  and  the  curve 
proceeds  smoothly.  As  discharge  goes  on  along  the 
curve  at  H,  diffusion  (now  of  acid  into  the  plate)  be- 
comes more  and  more  difficult.  The  active  concen- 
tration of  acid  droops,  and  at  the  point  I  the  cell  is 
for  practical  purposes  discharged.  Its  electromotive 
force  is  still  1.7  volts,  and  it  could  be  run  for  some 
time  longer  at  low  rates  before  dropping  to  zero.  As 
storage  batteries  are  used  in  practice,  1.7  may  be 
taken  as  the  limit  of  useful  discharge  at  a  low  rate. 
(See  page  118.) 

The  five  factors  of  page  27  are  just  as  important 
during  discharge  as  during  charge  and  for  the  reasons 
given  at  that  place.  Acid  density  determines  starting 
point  and  position  of  the  curve,  and  simultaneous  ex- 
amination of  discharge  voltage  and  density,  as  given 
in  the  curves  of  Figure  32,  enables  one  to  decide  upon 
the  condition  of  the  cell  as  to  charge  or  discharge 


CHAEGE  AND  DISCHARGE 


107 


12 


5«.M 

s   | 


1.9     12 


FIG.  32.  —  Discharge  curves  showing  change  in  acid  density  and  in 
voltage. 

from  acid  density  as  well  as  from  voltage.  Temper- 
ature affects  diffusion  and  therefore  acid  concentra- 
tion at  point  of  action  and  electromotive  force.  It 


CIRCU 


f  OP!  NED 


8  d  10  It  IZ          13  14          15  I  a  3  4 

MINUTES 

FIG.  33.  —  End  of  discharge  and  recovery. 


108 


STORAGE  BATTERIES 


also  affects  the  electromotive  force  directly.  Rate  of 
discharge  determines  acid  concentration  and  there- 
fore the  concentration  of  the  active  ions.  Type  of 


L30« 


125 


120 


1.15 


1.10 


1.00  I 


IS 


0.95 l 


X4- 


vro 


REACHES  L9 


IN  5HRS.2SMIN. 


10  ZO  30  40  50 

MINUTES 

FIG.  34.  —  Recovery  after  very  long  and  complete  discharge. 


plate  enters  and  previous  history  of  the  cell.     (See 
page  113.) 

68.  Recovery  after  Discharge.  —  The  curve  along 
which  recovery  takes  place  after  discharge  is  shown 
in  Figures  33  and  34.  It  is  very  much  like  a  diffu- 
sion curve,  and  represents  the  rate  of  return  to  the 


CHARGE  AND  DISCHARGE 


109 


normal  concentration  of  acid  in  the  cell  on  the  part  of 
the  acid  in  the  deep  interstices  of  the  plates.  It  is 
not  quite  the  right  shape  for  a  pure  diffusion  curve, 
and  the  equalization  of  concentrations  throughout 
the  cell  is  undoubtedly  assisted  by  local  action. 


1  2  3 

HOURS 

FIG.  35.  —  Charge  and  discharge  curves  of  [A]  Plante  and  [B]  mass 

plates. 

69.  Special  Peculiarities  of  Charge  and  Discharge 
Curves. — The  two  extreme  types  of  plate  — large  sur- 
face Plante  on  the  one  hand,  and  thick  mass  plates  on 
the  other  —  show  evident  differences  in  their  curves 
of  operation.  Figure  35  indicates  the  general  char- 
acter of  these  differences,  and  a  resume  of  the  theory 
of  the  inflections  of  these  curves  will  be  found  to 


110 


STORAGE  BATTERIES 


agree  with  the  physical  characteristics  of  the  plates. 
It  is  quite  possible  to  get  composite  curves  from 
composite  plates.  An  interesting  example  is  the 
type  of  ribbed  Plante  plate  now  very  common  all 
over  the  world  and  used  for  the  hardest  kind  of 


I 

514 


1.0 


0.8 


0.6 


15       30       45       60      75       90     105      120    135 
MINUTES 

FIQ.  36.  —  Full  discharge  curve  of  ribbed  Plant6  plate. 

work.  Figure  36  shows  the  full  discharge  curve  of 
a  Gould  plate.  For  the  greater  part  of  its  discharge 
it  behaves  like  a  large  surface  plate,  which  it  is. 
Then  the  action  reaches  that  part  of  the  plate  where 
there  is  a  considerable  mass  of  active  material,  much 
of  it  at  about  the  same  distance  from  the  main  bulk  of 
acid  in  the  cells.  Here  the  droop  is  stopped  for  a 
short  time,  and  only  when  the  action  has  penetrated 


CHARGE  AND  DISCHARGE 


111 


far  into  this  last  reservoir  of  material  does  the  final 
drop  begin.  And  the  final  drop,  instead  of  being 
like  that  of  a  large  surface  plate,  is  much  more  like 
a  mass  plate.  The  only  reason  why  these  peculiari- 
ties are  not  noticed  every  day  is  because  they  lie  at 


17 

1.6 
L5 

14 

13 
H 

I" 

c  „ 

d» 

o 

09 
0.8 
07 
0.6 
Q5 

\ 

N, 

X 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

\ 

x^ 

^"^ 

^ 

^ 

1         Z         34567         8         9        10.      II        12 

FIG.  37.  —  End  of  curve  of  complete  discharge  at  constant  rate  ( 
Plant6  type). 


voltages  lower  than  those  of  practical  service  condi- 
tions.    (See  also  Fig.  37.) 

70.  Charge  and  Discharge  at  Various  Rates. — Figures 
38  and  39  show  series  of  curves  of  charge  and  dis- 
charge for  two  types  of  plates  at  various  rates.  They 
hardly  require  detailed  discussion,  for  they  fit  very 
closely  the  general  principles  so  often  invoked  in 


112 


STORAGE  BATTERIES 


explanation  of  changes  in  cell  electromotive   force. 
The  charge  curves  have  much  the  same  general  char- 


FIG.  38.  —  Curves  of  operation  of  Plante  plates  at  various  rates. 

The  rates  for  the  curves  of  Figure  38  are 

For  8-hours  of  charge  or  discharge  1  ampere 
5  1.4    , 


3 

1 

20  minutes 
5  minutes 


2.0 

4.0 

8.0 

16.0 


These  are  the  rates  usually  specified  in  practice. 
The  capacities  corresponding  to  these  rates  are 

For  8-hour  charge  or  discharge  8  ampere-hours 


3    ,, 

1     „ 

20  minutes 

5  minutes 


7 

6 

4 

2.67 

1.33 


acteristics  at  different   rates,  but  show  more  rapid 
changes  as  the  rates  are  raised.     The  most  interest- 


CHARGE  AND  DISCHARGE 


113 


ing  thing  about  the  set  of  curves  is  the  information 
it  gives  about  the  last  factor  in  our  list  —  the  "  pre- 
vious history  "  of  the  cell.  It  makes  a  great  differ- 
ence in  the  discharge  curve  of  a  cell  whether  the  cell 
has  been  charged  at  a  high  or  a  low  rate,  and  just  as 
great  a  difference  in  the  charging  curve,  whether  the 


HOURS 

FIG.  39.  —  Curves  of  operation  of  mass  plates  at  various  rates. 

previous  discharge  has  been  fast  or  slow.  Take  a 
single  case.  Suppose  a  fully  charged  cell  has  been 
discharged  at  the  5-minute  rate.  It  is  evident  from 
the  figure  that  only  1.3  ampere-hours  have  been  drawn 
from  it.  We  only  need  to  return  a  little  more  than 
this  to  the  cell  to  charge  it  completely.  In  the  same 
way,  if  our  cell  has  been  completely  discharged  at  a 
low  rate,  and  then  charged  at  the  5-minute  rate,  we 
can  only  get  about  1.3  ampere-hours  into  it.  It  may 
be  fully  charged  for  a  5-minute  discharge,  but  it  is 


114 


STORAGE  BATTERIES 


by  no  means  fully  charged  for  a  3-hour  discharge. 
When  we  come  to  the  chapter  on  operation  we  shall 
have  another  side  of  this  same  problem  to  look  at  — 
the  one  which  deals  with  the  effect  of  charge  and  dis- 
charge rates  on  the  life  of  the  cell. 


CHARGE 

HARGE 


PL  Alt.  N.PI  W  IRKING       , 


DISCHAiiGE 


HOURS 

FIQ.  40.  —  Charge  and  discharge  curves.    Peroxide  and  lead  plates 
measured  against  an  auxiliary  electrode  (lead  plate). 

71.  Use  of  Auxiliary  Electrode. — It  is  very  fre- 
quently desirable  to  segregate  the  two  plates  in  a 
cell,  so  that  the  course  of  charge  and  discharge  may 
be  followed  for  each  separately.  Several  forms  of 
auxiliary  electrode  have  been  suggested,  and  the  one 
in  most  common  use  is  metallic  cadmium.  A  stick 
of  this  metal  is  used  as  one  electrode,  and  the  electro- 


CHAEGE  AND  DISCHARGE  115 

motive  force  Cd/dilute  Cd++  against  one  of  the  plates 
is  measured. 

It  is  evident  that  this  is  not  the  most  stable  of 
electrodes,  for  its  readings  are  dependent  on  the 
amount  of  current  flowing  through  the  cadmium  cir- 
cuit and  also  on  temperature  and  other  factors.  It 
answers  very  well  for  most  practical  purposes,  how- 
ever, and  some  of  the  curves  for  single  plate  poten- 
tials which  are  given  in  this  book  were  made  with 
its  aid. 

Another  way  of  following  the  single  electromotive 
forces  at  the  two  plates  is  to  use  an  idle  lead  or  per- 
oxide plate  as  a  third  electrode,  measuring  each  of 
the  working  plates  against  it.  Figure  40  gives 
charge  and  discharge  curves  for  working  positive 
and  negative  plates,  measured  against  an  idle  lead 
plate. 


CHAPTER  X 
CAPACITY 

In  our  observations  on  the  curves  of  charge  and 
discharge  we  found  that  at  least  five  factors  were 
active  in  fixing  the  shape  and  position  of  these 
curves.  These  same  factors,  together  with  the  limit 
of  voltage  set  by  practical  experience,  determine  the 
capacity  of  a  storage  cell  in  the  sense  in  which  this 
term  is  usually  applied. 

The  lower  limit  of  voltage  —  the  point  to  which 
the  cell  is  discharged  in  actual  service  —  is  not  by 
any  means  invariable.  At  low  rates,  as  in  telephone 
and  train  lighting  service,  it  is  about  1.8  volts.  In 
regulating  power  plant  loads,  and  in  much  of  the 
other  regular  work  which  a  battery  does,  it  is  about 
1.7.  At  very  high  rates,  as  when  an  emergency 
battery  is  called  upon  to  take  the  entire  load  of 
a  large  station,  it  may  be  carried  as  low  as  1 
volt.  Just  for  the  present  we  will  assume  1.7  volts 
as  the  limit  below  which  we  cannot  usefully  discharge 
our  cell,  and  we  will  base  its  capacity  on  this  point. 

72.  Faraday's  Law  and  Capacity.  —  Of  course  ca- 
pacity, in  the  basic  sense  of  the  word,  is  given  by 

116 


CAPACITY  117 

Faraday's  law,  and  can  be  calculated  directly  from 
the  equation 

Pb  +  Pb02  +  2  H2S04  =  2  PbS04  +  2  H2O. 

207  gm.  of  lead  sponge     1 

239  gm.  of  lead  peroxide  [•  give  2  x  96,540  coulombs, 

196  gm.  of  sulphuric  acid  J 

and  if  we  keep  the  current  small  enough,  it  might 

be  possible  to  get  this  theoretical  current  yield  at 

2  volts. 

Since  one  ampere-hour  is  3600  coulombs,  we  will 
need  for  one  ampere-hour,  3.86  gm.  lead,  4.45  gm. 
lead  peroxide,  and  3.6  gm.  H2SO4,  and  these  are  the 
amounts  of  active  materials  which  are  really  used 
up  in  any  storage  cell  during  the  passage  of  current 
to  the  amount  of  one  ampere-hour.  In  actual  prac- 
tice the  voltage  of  the  cell  would  have  fallen  to  zero 
long  before  all  the  material  in  the  plates  and  the 
electrolyte  had  been  acted  upon,  and  in  any  actual 
cell  there  is  always  a  very  large  excess  of  all  three 
of  the  constituents,  even  at  the  time  when  the  cell 
is  "discharged."  Besides,  there  must  always  be 
supports  for  the  active  lead  and  lead  peroxide,  and 
these  supports  must  in  practice  have  strength  and 
weight  enough  to  enable  them  to  withstand  many 
complete  cycles  of  charge  and  discharge.  As  we 
shall  see  later,  there  are  useful  types  of  cells  in 
which  the  materials  which  really  enter  into  reaction 


118 


8 TOE AGE  BATTERIES 


only  make  up  10  or  15  °/0  of  the  total  weight  of  the 
plates,  and  only  6  or  7  %  of  the  total  weight  of  the 
installation. 

73.  End  Voltage  determines  Capacity.  —  There  is  no 
doubt  whatever  about  our  oft-repeated  fundamental 
principle  that  it  is  the  acid  concentration  within  the 


\ 


I  Z  3  4  56789 

DISCHARGE  TIME- (HOURS') 

FIG.  41. —  Discharge  curves  at  various  rates. 

pores  of  the  plates,  at  the  point  where  the  action  is 
taking  place,  which  determines  the  voltage  of  the 
cell.  At  a  high  rate  of  discharge,  the  acid  density 
at  the  active  point  in  the  plate  is  low,  and  the  vol- 
tage curve  drops  after  a  comparatively  short  time. 
It  becomes  too  hard  for  the  electrolyte  to  get  to  any 
more  active  material,  even  though  there  is  plenty 
in  the  plates,  and  useful  discharge  must  be  stopped. 
Figure  41  gives  a  set  of  discharge  curves  made 


CAPACITY 


119 


in  actual  test  on  a  large  cell.  This  cell  was  charged 
each  time  at  a  constant  and  low  rate,  in  order  that 
the  charging  part  of  the  cycle  might  not  be  a  vari- 
able factor.  It  was  then  discharged  at  constant 
temperature  at  the  rates  given.  If  we  take  as  the 


I  £34  56789          10 

TIME- HOURS 

FIG.  42. —  Capacity  curves,  theoretical  (dotted),  and  experimental 
(full-line). 

voltage  for  stopping  discharge  1.70  for  most  of  the 
curves,  and  1.65  for  the  1  hr.,  and  1.6  for  the  20  min. 
discharges,  we  get  the  following  table  :  — 


CURRENT 

20  amp. 
40 
80 
1GO 


TIME  OF  DISCHARGE 

8  hours 
3 
1 
20  minutes 


CAPACITY  IN  A-H 

160  A-H 
120 

80 


120 


STORAGE  BATTERIES 


These  values  can  be  equally  well  expressed  by  means 
of  a  single  curve,  for  there  are  really  only  two  things 
to  be  related,  —  current  and  time.  The  expression  of 
the  capacity  in  a  separate  column  is  merely  for  the 
sake  of  having  a  direct  statement  of  capacity. 
Figure  42  contains  this  curve.  It  is  the  one  which 
is  drawn  as  a  full  line. 


2.0 


1.9 


1.8 


40 


50 


io  ao  30 

AMPERE-HOURS  DISCHARGED 

FIG.  43.  —  Discharge  curves  of  Plante  plates  at  the  1,  3,  and  8-hour 

rates. 

Figure  43  gives  discharge  curves  for  Plante  plates 
at  various  rates,  Figure  44  similar  curves  for  semi- 
Plante  plates,  and  Figure  45  curves  for  thick  mass 
plates.  In  the  three  cases  plates  were  chosen  with 
the  same  capacity  at  a  medium  rate  of  discharge  (3 
hours).  It  is  evident  that  the  large  surface  Plante 
plates  are  best  at  the  high  (1-hour)  rate,  and  that 
they  are  by  no  means  up  to  either  of  the  other  types 


20 


1.8 


X 


40 


50 


0  10  20  30 

AMPERE-HOURS  DISCHARGED 
FIG.  44.  —  Discharge  curves  of  semi-Plant6  plates  at  various  rates. 

at  the  low  (8-hour)  rate.     At  the  15-hour  rate  the 
curve  for  this  particular  plate  is  not  shown  in  the 


2.0 


1.8 


X 


X3 


40 


50 


10  £0  30 

AMPERE-HOURS  DISCHARGED 
FIG.  45.  —  Discharge  curves  of  thick  mass  plates  at  various  rates. 

figure.     It  would  reach  1.8  volts  at  about   32  on 
the  horizontal  axis. 


122 


STORAGE  BATTERIES 


The  mass  plates  of  Figure  45  are  very  short  of 
capacity  at  the  1-hour  rate,  but  they  are  far  better 
than  the  Plante  type  at  the  low  (15-hour)  discharge. 

The  semi- Plante  plate  lies  between  the  other  two. 


345676 

THICKNESS  IN  MILLIMETERS 


JO         II 


FIG.  46.  —  Capacity  as  a  function  of  the  thickness  of  a  paste  plate, 
at  various  rates.    Peroxide  plates  against  auxiliary  electrode. 

The  useful  end  voltage  has  been  placed  at  1.8  volts 
in  this  case. 

It  is  evident  from  these  curves  that  the  thickness 
(and  structure  generally)  of  a  plate  is  a  factor  of  im- 
portance in  its  working  capacity.  Experiments  with 
paste  plates  of  the  same  surface  and  varying  thick- 
ness give  the  results  shown  in  Figures  46  and  47,  the 
former  for  positive  plates  and  the  latter  for  negative. 


CAPACITY 


123 


The  five  curves  in  each  figure  are  for  different 
rates,  from  1  to  16  amperes. 


7 


7 


234567 

THICKNESS  IN  MILLIMETERS 


8          9 


FIG.  47.  —  Capacity  as  a  function  of  thickness  of  plate.    Negatives 
against  auxiliary  electrode. 

The  dotted  line  in  Figure  46  is  for  an  infinitely  slow 
rate  —  capacity  directly  proportional  to  thickness. 


124  STORAGE  BATTERIES 

74.  Formula  for  calculating  Capacity  at  Various  Rates. 
—  It  is  usually  possible  to  find  a  not  very  compli- 
cated mathematical  formula   to   fit   a   curve  which 
looks  like  Figure  42  and  the  dotted  line  in  this  figure 
is  plotted  as  an  expression  of  the  formula 

Int=  constant. 

n  for  this  particular  type  of  plate  is  1.45,  and  the 
constant  is  determined  by  putting  in  the  actual 
values  for  our  plate  at  one  rate  and  solving  the 
equation.  See  75,  below. 

This  exponent  n  is  rather  a  good  measure  of  the 
physical  qualities  of  a  plate.  It  is  large  for  thick, 
dense,  massive  ones  and  becomes  smaller  and  smaller 
as  the  plate  is  given  a  larger  surface  in  proportion 
to  its  content  of  active  material.  It  goes  as  high 
as  2.0  for  some  plates  of  the  most  thick  and  tender 
kind,  and  as  low  as  1.20  for  the  most  active  types  of 
large  surface  plates.  See  also  Figure  48.  A  little 
calculation  will  show  what  kind  of  a  family  of  dis- 
charge curves  at  different  rates  will  be  characteristic 
of  each  of  these  extremes.  The  one  with  exponent 
2.0  is  the  easiest  to  calculate. 

75.  Let  us  go  through  the  course  of  the  calculation 
of  such  a  curve  for  the  simple  case  where  n  =  2.0. 
Assuming  that  the  cell  gives  10  amperes  for  8  hr. 

^2£  =  constant 
102  x  8  =  constant 


CAPACITY 


125 


1.5 
1.4 
1.3 

1.2 
1.1 

1.0 
.9 

I" 
§' 

.6 
.5 
4 
.3 
.2 
.1 


!/ 


24          6          8          10         12          14         16          18        20 

HOURS  FOR  COMPLETE   DISCHARGE 

FIG.  48. 


126  STORAGE  BATTERIES 

Vt  =  800 

t  =  3          z2  x  3  =  800  ^2  =  267   i  =  16.3 
£  =  1  z2  =  800   ^=28.2 

t  =  J  ^2  =  2400   t  =  49 

t  =  98 


and  from  this,  when, 

current  is  =  10,  capacity  is  80. 

current  is  =  16.3,  capacity  is  49. 

current  is  =  28.2,  capacity  is  28.2. 

current  is  =  49,  capacity  is  16.3. 

current  is  =  98,  capacity  is    8.2. 

If  capacity  is  plotted  vertically  in  place  of  current, 
the  family  of  curves  for  various  exponents  becomes 
still  more  expressive.  Figure  48  gives  the  calculated 
curves  for  values  of  n  from  1.10  to  2.0. 

It  is  also  possible  to  derive  a  curve  like  the  one  in 
Figure  42  with  the  aid  of  the  theory  of  diffusion,  but 
the  assumptions  necessary  are  far-reaching,  and  the 
final  formula  is  in  fact  only  an  empirical  one  like  our 
own.  Diffusion  has  the  chief  role  to  play,  however, 
here  as  at  every  other  point  in  the  theory  of  the  lead 
(and  any  other)  accumulator. 

76.  Liebenow's  Diffusion  Experiment.  —  Liebenow, 
one  of  the  most  brilliant  of  the  students  of  the  lead 
cell,  made  an  interesting  experiment  on  the  effect  of 
merely  allowing  acid  to  flow  through  a  plate  which 
was  discharging.  His  arrangement  is  shown  in 


CAPACITY 


127 


Figure  49.  A  negative  plate  was  used  in  his  test,  and 
it  was  found  that  without  flow  it  gave  14.4  ampere- 
hours.  With  flow  it  gave  41.6  ampere-hours.  Such 
experiments  have  frequently  been  performed  of  late, 
and  it  is  a  most  interest- 
ing thing  to  see  a  plate 
which  has  been  ex- 
hausted without  flow,  so 
that  its  voltage  is  zero, 
pick  up  and  come  to  life 
again  as  soon  as  acid  be- 
gins to  flow  through  it. 
Its  voltage  rises  to 
nearly  1.7,  and  it  is  ca- 
pable of  doing  a  great 
deal  more  work. 

The  object  of  the  flow 
through  the  plate  is  to 
keep  the  acid  concentra- 
tion up  during  discharge 


FIG.  49.  —  Liebenow's  experiment 
to  show  the  effect  of  forcing  elec- 
trolyte through  the  plate  during 
operation. 


and  down  during  charge 

at  the  place  in  the  plate 

where  the  reaction  is  actually  taking  place.  Practi- 
cal applications  are  numerous.  Large  surface  plates 
are  necessary  where  charge  and  discharge  rates  are 
high.  They  contain  much  less  total  active  material 
than  paste  plates  of  the  same  weight,  but  the  material 
in  them  is  in  a  thin  layer,  and  diffusion  is  easy  to  all 


128  STORAGE  BATTERIES 

parts  of  it.  Then,  too,  thin  paste  plates  give  a  far 
larger  capacity  per  weight  than  thick  ones  operating 
on  the  same  rates  and  to  the  same  end  voltages. 
Aids  to  diffusion  are  perhaps  the  most  important 
improvements  which  can  be  made  in  storage  battery 
work  with  the  exception  of  the  all-important  one  of 
a  reasonably  long  life  under  hard  service  conditions. 

The  positive  plate  needs  help  more  than  the  nega- 
tive, for  besides  using  up  or  producing  sulphuric 
acid,  water  appears  or  disappears  at  that  point. 
It  will  be  seen  that  it  needs  help  1.6  times  as 
badly  as  the  negative.  In  spite  of  this  need  it  is 
harder  to  send  it  the  necessary  relief;  for  while 
negative  plates  can  be  made  both  tough  and  porous, 
the  positive  active  material,  lead  peroxide,  persists 
in  being  merely  a  dense  but  rather  loosely  inter- 
locked mass  of  fine  grains.  Some  rather  rough 
measurements  on  the  rate  at  which  acid  diffuses  into 
positive  and  negative  paste  plates  are  given  in  Figure 
50.  These  are  resting  plates,  however,  and  do  not 
take  into  account  the  greater  need  for  acid  of  the 
peroxide  plate  during  action. 

Lead  grows  on  the  negative  plate  as  real  trees  and 
sponges,  and  this  can  often  be  clearly  seen  in  vener- 
able negatives  on  which  the  lead  has  been  deposited 
and  redissolved  thousands  of  times.  The  positives  in 
the  same  cells  look  lean,  for  they  have  lost  much  of 
their  original  material,  and  if  they  are  healthy,  and 


CAPACITY 


129 


of  the  kind  that  have  proven  themselves  capable  of 
hard  work,  they  have  manufactured  more  active 
material  to  take  the  place  of  that  lost.  It  is  easy  to 
apply  Liebenow's  principle  to  the  negative  plate.  It 


CHAW 


io  ao 

MINUTES 


FIG.  50.  —  Diffusion  into  resting  positive  and  negative  plates. 

is  much  harder  to  persuade  acid  to  flow  through  one 
of  lead  peroxide. 

77.  Diffusion.  —  To  digress  for  a  moment  to  the 
general  subject  of  diffusion.  A  substance  in  solu- 
tion can  move  about  from  point  to  point  in  either 
of  two  ways  —  by  convection  or  by  diffusion.  The 
difference  in  velocity  with  which  a  given  amount 
of  a  substance  can  be  transported  from  "one  place 


130  STORAGE  BATTERIES 

to  another  by  the  two  methods  is  enormous.  Sup- 
pose a  tall  cylinder  with  a  couple  of  inches  of  a 
strong  solution  of  a  colored  salt  (copper  nitrate,  for 
example)  in  the  bottom,  and  with  pure  water  filling 
the  rest  of  the  cylinder.  By  convection  we  could 
mix  the  whole  to  a  homogeneous  average  solution 
in  ten  seconds,  by  violent  stirring  or  shaking.  By 
diffusion  alone  the  same  degree  of  mixing  would 
take  months. 

The  process  of  convection  could  be  delayed  in  the 
cylinder  by  filling  it  with  glass  or  cotton  wool.  In 
this  case  the  transfer  of  material  from  the  concen- 
trated solution  out  through  the  dilute  one  has  to 
take  place  through  spaces  in  the  inert  substances. 
It  is  much  as  though  the  cylinder  were  a  mile  long 
instead  of  a  foot.  Diffusion  will  also  be  delayed 
by  the  inert  filling,  but  in  much  less  degree.  The 
difference  becomes  still  more  evident  if  we  fill  the 
cylinder,  not  with  pure  water  solutions,  but  with 
solutions  which  set  to  a  jelly,  such  as  gelatine  or 
agar  —  a  concentrated  gel  below;  a  pure  water  gel 
above.  Now  convection  is  entirely  stopped  and 
diffusion  has  all  the  work  of  transportation  to  do. 
The  process  becomes  a  very  tedious  one  indeed. 

78.  Diffusion  and  Convection  in  the  Cell.  —  In  the 
storage  battery  the  real  transport  of  all  material  is 
a  matter  of  diffusion.  Solid  material  is  there  in 
plenty,  but  the  acid  of  the  electrolyte  is  just  as 


CAPACITY  131 

necessary  for  the  reaction  as  the  solids,  and  it  has 
to  come  to  the  solid  by  diffusion  through  the  fine 
pores  of  the  active  material.  At  certain  portions 
of  the  cell  cycle  convection  comes  along  to  help, 
especially  when  gas  is  being  evolved  in  the  plates. 
The  gas  bubbles  stir  everything  up  and  assist  greatly 
in  bringing  materials  to  the  point  where  they  are 
needed.  The  difference  in  density  between  the  con- 
centrated acid  formed  during  charge  and  the  aver- 
age acid  of  the  cell  also  gives  rise  to  convection 
currents,  which  can  be  clearly  seen  by  looking  across 
the  face  of  a  plate  toward  a  bright  source  of  light. 
If  the  cell  is  charging,  a  thin  stream  of  denser  elec- 
trolyte can  be  seen  running  down  the  face  of  the  plate 
and  curling  up  on  the  bottom  of  the  cell.  The  more 
dilute  acid  can  also  be  seen  rising  up  along  the  face 
of  the  plate  during  discharge. 

79.  Recovery  Curves  and  Diffusion  Curves.  —  The 
curves  in  Figures  30,  33,  and  34  are  very  nearly  like 
diffusion  curves.  When  the  circuit  is  closed  for  dis- 
charge, material  is  rapidly  exhausted  near  the  solid 
particles  which  are  active.  The  concentration  gradi- 
ent becomes  steep  and  acid  begins  to  diffuse  toward 
that  point.  Lead  sulphate  is  formed  in  the  solution 
and  presently  a  state  of  very  dynamic  equilibrium  is 
reached.  Acid  is  being  transported  by  diffusion  just 
fast  enough  to  supply  the  demand  at  the  point  of 
reaction  ;  and  lead  sulphate  is  being  removed  by  pre- 


132  STORAGE  BATTERIES 

cipitation  as  fast  as  it  is  formed.  The  curves  re- 
ferred to  are,  of  course,  voltage  curves,  but  the 
relations  of  page  92  show  clearly  that  the  curves 
can  equally  well  express  the  average  concentration 
of  reacting  materials  at  the  point  of  action.  The 
recovery  curve  of  page  133  is  of  the  same  nature.  At 
the  lower  part,  at  the  beginning  of  the  recovery  curve 
in  Figure  33,  we  have  the  final  condition  described 
above.  Materials  are  being  supplied  at  a  rate  just 
able  to  maintain  the  concentration  at  a  rather  low 
and  constantly  decreasing  value.  When  the  circuit 
is  opened,  consumption  of  material  ceases.  But  the 
concentration  at  the  point  where  the  reaction  was 
going  on  was  different  from  that  outside  in  the  body 
of  the  cell.  Diffusion,  therefore,  continues  and  the 
concentration  differences  become  smaller  until  diffu- 
sion becomes  indefinitely  slow. 

Theoretically  these  curves  take  an  infinite  time  to 
become  perfectly  flat,  but  practically  they  approach 
very  near  to  a  final  value  within  a  few  minutes.  One 
exception  to  this  last  statement  will  occur  to  every 
one  who  watches  storage  cells  closely.  A  very  fully 
charged  cell,  which  has  been  gasing  freely,  takes  a 
long  time  to  return  to  its  open  circuit  electromotive 
force  (see  Fig.  51).  This  cannot  be  due  to  any  high 
concentration  of  acid  in  the  pores  of  the  plates,  for 
practically  all  the  materials  have  long  since  been  dis- 
posed of  and  only  an  infinitesimal  amount  of  acid  is 


CAPACITY 


133 


being  produced.  There  is  another  reason  for  this 
slow  approach  to  the  normal  open-circuit  voltage. 
At  the  end  of  full  charge,  practically  all  the  dissolved 
sulphate  has  been  driven  out  of  solution.  Opening 
the  circuit  at  the  end  of  such  a  charge  permits  lead 


2JC 


16        18        ZO        iZ        24- 


FIG.  51.  —  Recovery  curve  after  complete  charge. 

sulphate  to  form.  Local  action  takes  place  at  the 
places  where  support  and  active  material  are  in  con- 
tact. So  lead  sulphate  is  soon  present  inside  the 
plate.  But  before  it  reaches  its  normal  maximum 
concentration  at  all  points  in  the  plate  it  has  to 
saturate  the  entire  electrolyte.  The  drop  in  voltage 
is  therefore  not  so  rapid  as  it  would  be  if  only  acid 
diffusion  were  to  be  considered.  Besides  the  diffu- 
sion of  an  already  dissolved  substance,  we  have  to 
wait  in  this  case  for  its  formation. 


134 


STORAGE  BATTERIES 


80.  The  Effect  of  Temperature  on  Capacity.  —  Since 
capacity  is  determined  by  a  fixed  voltage  limit  as 
well  as  by  other  factors,  we  must  expect  to  find  that 
the  effect  of  temperature  will  be  a  considerable  one. 
Figure  52  gives  a  set  of  discharge  curves  at  the  same 
rate  but  at  the  different  temperatures  indicated  on 


\ 


135* 


90 
TIME- MINUTES 


150 


FIG.  52.  —  Discharge  curves.    All  made  at  same  rate  but  at  various 
temperatures. 

the  curves.  This  was  taken  with  constant  charge 
conditions.  The  cell  was  in  every  case  charged  at 
25°  C.  Its  temperature  was  then  changed  by  heat- 
ing or  cooling  the  thermostat  in  which  it  was  kept, 
and  after  remaining  constant  for  five  or  six  hours, 
charging  at  a  low  rate  all  the  time,  the  discharge  was 
taken.  The  rate  was  such  as  should  give  complete 


CAPACITY  135 

discharge  in  one  hour  under  normal  conditions  of 
service,  and  the  25°  curve  shows  this.  The  voltage 
dropped  to  1.7  in  just  about  one  hour.  At  48°  the 
same  cell  ran  for  an  hour  and  three  quarters;  at  8° 
for  half  an  hour.  A  difference  of  over  100  %  for 
quite  possible  limits  of  temperature,  and  of  over 
300  %  within  temperatures  not  really  dangerous  to 
the  life  of  these  cells ! 

This  is  a  very  high  temperature  coefficient,  to  be 
sure,  but  it  is  hardly  possible  to  make  a  cell  which 
has  not  a  coefficient  of  at  least  one  per  cent  per 
degree  in  the  ordinary  working  range  of  tempera- 
tures. 

Everything  combines  to  make  the  storage  cell  work 
better  and  more  efficiently  at  the  higher  temperature. 
For  the  usual  acid  concentration  the  temperature  co- 
efficient of  electromotive  force  is  positive,  and  has  a 
value  not  far  from  0.0003  volt  per  Centigrade  degree. 
This,  of  course,  has  nothing  to  do  with  the  ampere- 
hour  capacity  of  the  cell,  except  to  raise  the  voltage 
a  little,  and  thus  lengthen  the  time  of  discharge  a 
little.  Examination  of  the  discharge  curves  at 
various  temperatures  will  show  how  very  little  this 
affects  the  total  number  of  ampere-hours  which  can 
be  taken  from  the  cell.  A  difference  of  30°  C.  means 
0.0003  x  30,  or  a  rise  of  only  0.009  volt  in  the 
fundamental  cell  electromotive  force  due  to  the 
higher  temperature,  and  this  is  not  even  measurable 


136  STORAGE  BATTERIES 

on  a  curve  which  is  drooping  as  rapidly  as  the  low- 
temperature  curves  of  Figure  52. 

81.  Reaction  Velocity.  — But  the  other  two  factors 
are  highly  important.     One  of  these  is  the  diffusion, 
which  we  have  discussed  at  length.     The  other  is  not 
less   important,    probably,   though  it  is  much  more 
difficult  to  isolate  and  examine.     This  is  the  increased 
reaction  velocity.     Whatever  the  reactions  which  are 
basic  for  the  action  of  the  cell,  we  have  found  very 
good  evidence  that  the  transport  through  the  elec- 
trodes is  cared  for  by  ions  which  are  present  in  very 
small  concentration. 

The  velocity  with  which  these  ions  are  formed  from 
the  solid  material  of  the  plates,  in  reaction  with  the 
electrolyte,  is  a  determining  factor  of  importance. 
As  a  matter  of  fact  the  temperature  effect  on  the  cell 
is  too  great  to  be  ascribed  to  diffusion  alone.  And 
while  in  most  cases  reactions  between  ions  take 
place  so  rapidly  that  they  are  quite  unmeasurable,  it 
is  not  impossible  that  the  effect  should  be  evident  in 
such  a  case  as  this,  where  the  ionic  concentrations 
are  so  very  small. 

82.  Effect  of  Acid  Density  on  Capacity.  —  Measure- 
ments of  the   capacity  of  a  cell  with  varying  acid 
density,  and  with  all  the  other  factors  which  might 
affect  its  behavior  kept  as  constant  as  possible,  give  a 
very  simple  and  interesting  result.     The  cell  shows 
its  maximum  of  capacity  for  an  acid  of  maximum 


CAPACITY 


137 


conductivity.  This  is  in  both  cases,  for  sulphuric  acid, 
of  density  about  1.22.  (See  Figure  53.)  We  shall 
be  better  able  to  explain  the  reason  for  this  coinci- 
dence when  we  have  discussed  the  facts  about  the 


10'  15*  aO°  25°  30"  35* 

ACID  DENSITY 

FIG.  53.  —  Change  in  cell  capacity  at  various  rates  (1,  2,  4,  8,  and  16 
amperes)  with  various  acid  concentrations.  (See  also  Figures  72 
and  73). 

internal  resistance  of  our  cell,  and  we  will  therefore 
leave  it  until  we  reach  that  chapter  (page  167). 

83.  Watt-hour  Capacity.  —  It  is,  of  course,  the 
energy  capacity,  or  watt-hour  capacity,  of  the  cell 
which  really  interests  us.  This  is  found  by  multi- 
plying the  ampere-hour  capacity  by  the  average  vol- 
tage of  discharge.  The  curves  of  Figure  54  are  the 
same  as  those  of  Figure  41,  and  on  each  a  straight 


138 


STORAGE  BATTERIES 


line  was  laid  out  along  the  average  cell  electro- 
motive force  during  the  time  of  discharge.  The 
areas  under  these  lines,  including  everything  from 
time  zero  to  time-end  of  discharge,  and  from  the 
line  of  average  electromotive  force  down  to  zero 
electromotive  force,  give  watt-hours  if  we  multiply  in 


3456 
HOURS  OF  DISCHARGE 

FIG.  54. — Watt-hour  capacity  areas  at  various  rates,  and  discharge 
curves  from  which  they  were  taken.  Discharge  at  1,  1.4,  2,  and  4 
amperes. 

each  case  by  the  discharge  current.  The  rectangles 
give  the  set  of  areas  so  produced,  merely  as  visual  in- 
dication of  the  variation  in  energy  capacity  of  a 
storage  cell  with  change  in  discharge  current.  The 
same  differences  are  given  in  Figure  55  for  tempera- 
ture variation,  for  one  type  of  cell  only.  Other 
curves  for  these  same  relations  will  be  found  on  page 


CAPACITY 


139 


253,  in  the  discussion  of  various  types  of  cells  under 
actual  working  conditions. 

84.  Weight  Capacity.  —  For  most  purposes  where 
the  battery   has   to    be    carried    about   the    energy 
capacity  per  pound  of   battery  is  a  very  important 
ratio.     This  is  especially  true  of  batter- 
ies which  are  used  for  electric  vehicles, 
and  for  submarine  boats.     The  calcula- 
tion of  this  factor  is  very  simple.    Divide 
the  total  watt-hour  output  of 
the  battery  at  the  desired  rate 
by    the   total   weight   of   the 
battery    and    con- 
nections.     Data    on 
actual  tests  will  be 
found   in   chapter 
XVIII,  page  254. 

This  factor  is  not 
one  of  much  interest 
to  the  buyer  of  a  large  stationary  battery,  but  it  is  a 
matter  of  interest  to  the  manufacturer  who  has  to 
pay  for  the  lead  used  in  making  the  battery,  and 
therefore  has  a  good  deal  to  do  with  the  price  which 
he  is  obliged  to  ask  for  a  battery  to  do  a  certain  kind 
of  work.  The  modern  tendency  to  install  paste 
plates  in  large  emergency  batteries  is  a  good  example 
of  this  fact.  The  paste  plates  give  a  much  larger 
watt-hour  efficiency  per  pound  of  total  battery,  and 


8°C 


25°C 


48°C 


FIG.  55.  —  Watt-hour  capacity  areas  at 
various  temperatures. 


140  STORAGE  BATTERIES 

as  they  are  also  much  cheaper  to  make  per  killo watt- 
hour,  they  can  be  sold  cheaper  than  the  large-surface 
plates  of  the  same  total  capacity.  It  becomes  merely 
a  question  of  life  and  cost  of  maintenance  whether 
this  type,  or  the  perhaps  longer-lived  Plant£  plates, 
shall  be  used  for  this  work. 


CHAPTER  XI 

EFFICIENCY 

85.  There  are  two  ways  of  stating  what  is  called 
the  efficiency  of  a  storage  cell.     One  of  these  is  in 
terms  of  ampere-hours;  it  is  the  ratio  of  the  num- 
ber of  ampere-hours  which  can  be  taken  out  of  the 
cell  to  the  number  which  must  be  put  into  it  to  bring 
it  back  to  its  original  condition.    The  other  efficiency 
is  expressed  in  terms  of  watt-hours  —  the  ratio  of  the 
watt-hours  taken  out  to  those  put  in.    The  first  kind  of 
efficiency  is  more  or  less  misleading  as  a  criterion  of 
the  quality  of  a  cell,  but  the  second  is  of  decided 
interest  and  importance. 

86.  Ampere-hour  Efficiency.  —  From  what  we  have 
already  said  about  the  behavior  of  a  cell  in  charge 
and   discharge  it  is  evident   that  the   ampere-hour 
efficiency  of   most  cells  under  the  usual  conditions 
will  be  high  —  it  will  be  nearly  100%.     For  the  only 
way  in  which  current  is  lost  is  by  local  action  and  by 
the  evolution  of   gas  during  charge.     If   charge  is 
carried  on  at  a  very  low  rate,  gas  does  not  begin  to 
form  on  the  plates  until  very  near  the  end  of  charge. 
The  DE  part  of  the  charge  curve  (see  Figure  27) 

141 


142  STORAGE  BATTERIES 

is  steep  and  occupies  only  a  .small  fraction  of  the 
whole  time.  Gas  begins  to  form  rather  suddenly, 
and  at  this  time  the  cell  is  practically  fully  charged. 
Under  these  conditions  the  ratio 

ampere-hours  taken  out 
ampere-hours  put  in 

is  very  nearly  unity. 

Even  at  fairly  high  rates  the  production  of  gas 
only  involves  the  expenditure  of  a  comparatively 
small  fraction  of  the  current  sent  into  the  cell,  and 
for  working  charge  rates  it  leads  to  ampere-hour 
efficiencies  of  90%  to  95%. 

The  losses  due  to  local  action  are  very  small  if  the 
cell  is  charging  and  discharging  with  only  a  small 
interval  of  rest.  And  this  is  usually  the  case  where 
efficiency  is  a  factor  of  importance.  If  a  battery  is 
standing  on  open  circuit  for  a  long  time,  with  only 
an  occasional  charge  to  keep  it  in  good  condition,  and 
with  a  rare  discharge  at  a  very  high  rate  (as  in  the 
case  of  a  stand-by  or  emergency  battery),  efficiency  as 
such  is  not  a  factor  which  need  be  considered  at  all. 
The  interest  on  the  battery  investment  on  this  latter 
case  is  so  much  greater  than  all  the  coal  expended 
on  it  that  the  latter  item  disappears  completely. 
The  factor  which  is  of  importance  in  such  an  emer- 
gency battery  is  watt-hour  capacity,  and  if  this  could 
be  attained  conveniently  with  a  cheap  battery  of 


EFFICIENCY  143 

efficiency  20%,  we  would  see  this  type  of  battery 
installed  in  stations  which  require  this  kind  of  "  in- 
surance." 

Formally,  ampere-hour  efficiency  is 


^charge  ^charge 

and  for  most  purposes  in  service  it  will  be  found  to 
be  from  90  %  to  95  % .  As  far  as  this  is  concerned 
the  battery  is  about  as  efficient  as  any  of  the  ordinary 
electrical  machinery. 

87.  Energy  Efficiency.  —  The  other  and  more  im- 
portant kind  of  efficiency  is  energy  efficiency,  and 
this  is  the  ratio  of  the  energy  which  can  be  taken 
from  the  cell  to  that  put  into  it.  Or, 

watt-hours  taken  out 


watt-hours  put  in 
This  is  also  evidently  expressible  as 


where  i  and  t  have  the  same  meaning  as  above  and 
ec  and  ed  are  the  average  cell  voltages  of  charge  and 
discharge  respectively. 

88.  Data  for  Efficiency  Calculation.  —  The  most  direct 
way  to  get  data  on  the  value  of  EE  is  for  us  to  ex- 
amine sets  of  curves  like  those  in  Figure  41  and 
Figure  53  and  calculate  ampere-  and  watt-hour 


144 


STORAGE  BATTERIES 


4.0 


2.0 


1.4 


1.0 


FIG.  56.  —  Ampere-hour  efficiencies  at  various  rates.    Plant6  plates 
discharged  at  1,  1.4,  2,  and  4  amperes.    Charge  at  1  ampere. 

efficiencies  from  them.     Figures  56  and  57  give  areas 
so  calculated  from  a  similar  set  of  charge  and  dis- 


4.0 


2.0 


1.4 


1.0 


FIG.  57. —  Watt-hour  efficiencies  at  various  rates.  Plant6  plates  dis- 
charged at  1,  1.4,  2  and  4  amperes.  Charge  at  same  rate  as  dis- 
charge. 


EFFICIENCY 


145 


charge  curves.  It  will  be  noticed  that  while  the 
ampere-hour  efficiencies  are  good  enough  even  at  the 
higher  rates,  the  watt-hour  efficiencies  fall  off  pretty 
rapidly,  going  as  low  as  60  %  at  the  highest  rates  of 
charge  and  discharge.  These  are  rather  extreme 
cases,  however,  for  storage  cells  in  hard  service  are 


4.0 


2.0 


1.4 


.0 


FIG.  58. 


rarely  charged  as  fast  as  they  are  discharged,  and  the 
actual  figures  are  a  little  higher  than  those  obtained 
by  holding  rigorously  to  a  charge  rate  as  high  as  that 
of  discharge.  This  will  be  very  evident  if  we  take  a 
medium  rate  for  charge  and  determine  efficiency  for 
this  rate  and  various  discharge  rates.  Figure  58  gives 
these  data.  Here  we  have  assumed  the  one-hour  rate 
of  charge,  and  taken  the  corresponding  curve  through- 


146 


STORAGE  BATTERIES 


out.  At  very  low 
rates  the  charge 
and  discharge  vol- 
tages may  be  nearly 
the  same  through- 
out the  whole  cycle 
of  operation.  Fig- 
ure 59  shows  the 
change  in  cell  vol- 
tage at  the  various 
low  charge  and  dis- 

FIG.  59.  — Charge  and  discharge  voltages     charge  rates  ffiveil. 
at  very  low  rates. 

At  the  lowest  rates 
the  cell  shows  an  efficiency  of  nearly  100  %. 

Figure  60  shows  charge  and  discharge  voltage  at 
practical  rates. 

In  batteries 
which  are  worked 
severely  every  day 
and  all  day,  at 
rates  which  aver- 
age perhaps  as 
high  as  the  one- 
hour  rate  of  dis- 
charge, the  matter 
of  efficiency  is 
worth  careful  con- 
sideration. Under 


J 


z       i       o 

CURRENT  DENSITY 


«-  DISCHARGE          CHARGE  - 

FIG.  60.  —  Average  voltages  of  charge  and 
discharge  at  various  practical  rates. 
Plante  cells. 


EFFICIENCY  147 

these  circumstances  the  difference  in  the  coal  bill  for 
an  efficient  and  an  inefficient  battery  may  be  of  the 
same  order  as  the  depreciation  and  maintenance  of 
the  battery  for  the  same  length  of  time.  In  vehicle 
batteries  which  are  worked  on  regular  runs  leading 
to  a  full  discharge  every  day  or  oftener,  the  same 
relations  will  be  found  to  hold.  A  difference  of 
10  %  in  watt-hour  efficiency  will  be  of  the  same  im- 
portance in  dollars  and  cents  as  the  depreciation  on 
the  battery  for  the  year.  It  is  on  such  points  as 
this  that  choice  must  be  made  between  two  types  of 
battery.  The  battery  with  the  slightly  higher  de- 
preciation or  shorter  life  is  sometimes  to  be  chosen 
for  the  sake  of  the  saving  which  can  be  made  with  it 
on  account  of  its  higher  watt-hour  efficiency.  We 
can  of  course  discuss  matters  of  price  and  cost  only 
in  the  most  general  way,  but  we  shall  often  have 
occasion  to  call  attention  to  points  like  this. 


CHAPTER   XII 
INTERNAL  RESISTANCE 

89.  Practical  Cells.  —  The  internal  resistance  of  a 
storage  cell  of  commercial  dimensions  is  very  small 
indeed  and  may  frequently  be  entirely  neglected  in 
calculations  on  the  circuit  containing   a  battery  of 
cells.     Even  in  small   portable   cells  the  resistance 
seldom  rises  above  0.05  ohm  and  in  large  stationary 
cells  it  may  be  as  small  as  a  few  hundred-thousandths 
of  an  ohm. 

90.  Specific  Resistance.  —  In  calculating  and  stating 
the  resistance  of  a  substance  we  always  take  as  refer- 
ence a  cube  of  the  substance  1  cm.  on  an  edge,  with 
electrodes  covering  the   two   opposite   faces.     This 
specific  resistance  once  known,  we  can  calculate  the 
resistance  of  a  wire  of  any  size  or  length  made  from 
the  same  material. 


where  K  is  the  specific  resistance,  I  is  the  length,  and 
q  the  area  of  the  cross-section  of  the  conductor,  and 
R  is  the  required  resistance. 

The  table  on  page  263  gives  the  specific  resistance 
148 


INTERNAL  RESISTANCE 


149 


of  some  important  substances.  All  pure  metals  have 
positive  temperature  coefficients  —  they  increase  their 
resistance  when  they  are  heated.  All  electrolytes, 
on  the  contrary,  decrease  in  resistance  with  rise  of 
temperature.  An  alloy  may  behave  in  either  way  or 


R 

10 

LJ 

6 

l« 

1 

1 

I 

f 

\ 

/ 

/ 

\ 

^ 

/ 

^^ 

^~~~^ 

10                ZO               30                40               50               60                70              60               9C 

PERCENTAGE  OF  H2S04  IN  SOLUTION 

FIG.  61.  —  Specific  resistance  of  sulphuric  acid  solutions  containing 
varying  percentages  of  1.842  acid. 

may  have  a  positive  coefficient  at  one  temperature 
and  a  negative  one  at  another. 

In  the  storage  cell  the  solid  substances  all  have 
positive  coefficients  like  metals.  The  electrolyte  is 
of  course  a  member  of  the  other  class.  The  specific 
resistance  of  sulphuric  acid  of  various  concentrations 
is  given  in  Figure  61. 

For  many  calculations  it  is  more    convenient   to 


150  STORAGE  BATTERIES 

use  the  reciprocal  of  the  resistance,  the  conductance, 
and  the  corresponding  specific  conductance.  The 
conductance  of  electrolytes  forms  one  of  the  most 
interesting  chapters  of  general  electrochemistry,  but 
we  shall  not  have  occasion  to  use  many  of  its  prin- 
ciples, and  it  must  therefore  be  looked  up  in  some 
other  book. 

Unit  conductance  and  unit  resistance  refer  to  the 
same  thing.  A  wire  with  resistance  100  ohms  has 
conductance  0.01,  and  so  forth. 

91.  Acid  Resistance  in  the  Cell.  —  Let  us  calculate 
the  approximate  resistance  of  the  electrolyte  alone 
in  some  cells  of  very  different  size.  First,  a  spark- 
ing cell  with  three  plates  each  3  in.  square 
(7.6  x  7.6  cm.)  and  0.4  in.  apart  (1  cm.).  The 
total  acid  area  is 

7.6  x  7.6  x  2  =  115  sq.  cm. 

The  specific  resistance  of  sulphuric  acid  of  cell 
strength  is  about  1.5,  and  since  the  plates  are  about 
1  cm.  apart,  the  resistance  of  the  cell  will  be 

1.5  X=  0.013  ohm. 


The  second  calculation  will  be  for  a  fairly  large  cell 
such  as  would  be  used  in  a  regulating  battery.  It 
contains  thirty-one  plates,  each  15  in.  square  and 
with  0.4  in.  separation.  The  acid  area  is  in  this  case 

42  x  42  x  30  cm.  =  17,000  sq.  cm., 


INTERNAL  RESISTANCE  151 

and  the  acid  resistance  of  the  cell  is 

L5x 


or  a  little  less  than  0.0001  ohm. 

About  the  largest  cells  which  are  in  common  use 
have  perhaps  131  plates  about  15  x  30  in.  In  such 
a  cell  the  acid  area  is  therefore  about  290,000  sq.  cm. 
and  the  acid  resistance  is  about  0.000005  ohm. 

92.  Acid  Resistance  and  Temperature.  —  The  change 
of  resistance  of  the  cell  acid  with  temperature  is 
shown  in  the  dotted  curve  of  Figure  62,  and  it  is 
also  given  quite  accurately  by  an  equation  of  the 
form 


where  a  and  0  are  calculated  from  measurements 
made  at  two  temperatures. 

93.  Acid  Resistance  and  Cell  Losses.  —  It  may  be 
taken  as  an  approximate  general  statement  that  the 
total  internal  resistance  of  a  cell  is  about  double  the 
acid  resistance.  This  approximation  is  usually  suf- 
ficiently close  to  be  useful  in  the  calculation  of  losses 
inside  the  cell  due  to  resistance. 

Suppose  we  are  drawing  an  average  current  of 
2000  amperes  from  our  biggest  cell  just  considered. 
The  losses  in  the  cell  are 

i*r=  2000  x  2000  x  0.00001, 


152 


STORAGE  BATTERIES 


.07, 


04 


.03 


80°  30°  40°  50°  60C 

TEMPERATURE 


FIG.  62.  —  Change  in  resistance  of  cell  acid  with  temperature  (dotted 

line). 


INTERNAL  RESISTANCE 


153 


40  watts  in  all.  The  cell  is  furnishing  2000  x  1.8  = 
3600  watts,  and  our  resistance  loss  is  therefore  just 
about  \%.  This  is  so  small  in  comparison  with  the 
normal  working  losses  of  the  cell  at  this  rate  (about 
25%)  as  to  be  negligible. 


y.06 


35> 


eo  90  iao 

TIME  OF  DISCHARGE  (IN  MINUTES) 


(80 


FIG.  63. — Resistance  curves  of  Plant6  cell  during  discharge  at  va- 
rious temperatures. 

94.  Resistance  Curves.  — It  is  quite  true  that  the 
internal  resistance  of  a  storage  cell  is  usually  negli- 
gible as  far  as  loss  of  energy  is  concerned.  There 
are,  however,  many  things  of  great  theoretical 
(and  therefore  practical)  interest  about  this  factor. 
Hardly  anything  about  a  lead  cell  gives  so  clear 
an  insight  into  its  internal  workings  as  its  internal 
resistance.  Even  its  voltage  curve  cannot  tell  more 


154 


STORAGE  BATTERIES 


about  the  minute  phenomena  of  charge  and  dis- 
charge than  can  be  seen  from  its  resistance  curve. 
Figure  63  gives  a  set  of  curves  of  resistance  taken 
during  the  discharge  of  a  Plante  cell  at  various  tem- 
peratures, and  Figure  64  gives  both  voltage  and 


RESISl 


ANCE 


TIME-MINUTES 

FIG.  64.  —  Curves  of  resistance  and  voltage  during  complete  discharge 
and  partial  reversal  of  a  Plant6  cell. 

resistance  for  the  same  cell  at  one  temperature.  It 
will  be  noticed  that  the  change  in  resistance  is  con- 
siderable, if  the  cell  is  discharged  down  below  its 
usual  end  voltage  —  say  down  nearly  to  zero.  Fig- 
ure 65  gives  voltage  and  recovery  curves  during  par- 
tial discharge  and  recovery  curves  after  open  circuit 
immediately  following  the  discharge. 


INTERNAL  RESISTANCE 


155 


95.  Factors  of  Resistance.  —  The  total  cell  resistance 
is  evidently  made  up  of  at  least  three  distinct 
parts  as  indicated  in  the  diagram  of  Figure  66 :  — 

A.    Support  plate. 


L7 


g-06 

I 

05 


20 


100 


120 


VO  60  SO 

TIME-HINUTES. 

FIG.  65.  —  Curves  of  resistance  and  voltage  during  discharge  and  re- 
covery.   Plante  cell. 

B.  Active  material,  including  electrolyte  in  the 
pores. 

(7.    Main  body  of  electrolyte. 

A  and  0  we  can  consider  practically  constant,  and 
if  O  changes,  we  can  calculate  the  amount  of  the 
change  from  the  data  of  Figure  61,  which  gives  the 
relation  between  resistance  and  acid  concentration. 
B  is  the  variable  part  of  the  system. 


156 


STORAGE  BATTERIES 


During  charge  the  active  material  first  to  react  is 
near  the  surface  of  the  plate,  and  the  electrolyte  does 
not  have  to  diffuse  far  through  the  narrow  channels 
of  the  mass.  As  the  diffusion  path  increases  and 
the  cell  becomes  more  fully  charged,  concentrated 
acid  is  produced  in  the  pores.  But  all  through  the 


FIG.  66.  —  Diagram  of  the  parts  of  a  Plante  cell. 

charge  it  is  the  solid  plate  itself  which  does  most  of 
the  conducting,  and  the  change  of  resistance  to  be 
expected  during  charge  is  therefore  not  great. 

During  discharge  a  very  different  state  of  affairs 
exists.  In  this  case  also  the  action  begins  at  the 
surface,  where  there  is  plenty  of  both  electrolyte 
and  active  material.  But  as  discharge  proceeds  the 
area  of  activity  moves  back  deeper  into  the  mass, 
acid  is  used  up  within  the  plate  and  must  be  replaced 
by  diffusion.  The  acid  concentration  becomes  much 


INTERNAL  RESISTANCE  157 

lower  at  the  point  of  activity,  and  there  is  added  to 
this  the  loss  of  conductivity  by  the  solid  plate  itself. 
The  particles  of  lead  and  lead  peroxide  in  the  outer 
layers  have  now  become*  covered  with  a  layer  of  lead 
sulphate  and  have  been  more  or  less  insulated  from 
each  other.  The  result  is  as  if  the  distance  between 
the  plates  had  been  increased,  for  the  plate  surface 
which  is  actually  carrying  the  current  has  moved 
from  the  surface  back  into  the  interior  of  the  plate. 
The  surface  of  the  plate  in  contact  with  electrolyte 
has  also  been  greatly  decreased  by  this  displacement 
of  the  active  plate  surface. 

Such  changes  as  these  are  quite  sufficient  to 
account  for  the  change  found  in  the  resistance  of 
cells  under  the  usual  conditions  of  charge  and  dis- 
charge. We  should  not  expect,  and  we  do  not  find, 
any  very  large  or  very  rapid  changes  in  cell  resistance. 

96.  Sulphation.  —  On  long  standing,  a  storage  cell 
may  acquire  a  very  high  resistance  indeed  as  the  re- 
sult of  complete  "sulphation."  This  means  that  the 
active  lead  sulphate  formed  during  normal  discharge 
has  gradually  changed  into  the  inactive  crystalline 
form,  and  that  crystals  of  this  inactive  modification 
have  completely  covered  the  particles  of  lead  and 
lead  peroxide  with  an  insulating  coating.  Authentic 
cases  are  known  of  large  cells  with  internal  resist- 
ance as  high  as  10  ohms. 

As  usual,  it  is  hard  to  make  things  act  properly 


158  STORAGE  BATTERIES 

when  you  want  them  to.  I  have  left  a  completely 
discharged  cell  for  six  weeks  or  more,  carefully  fol- 
lowing its  internal  resistance  every  day,  and  found  no 
change  of  more  than  a  few  per  cent  in  its  resistance. 
It  seems  very  likely  that  the  ordinary  cases  of  sul- 
phation,  which  are  rather  common  and  most  annoy- 
ing in  their  results,  do  not  lead  so  much  to  a  very 
high  internal  resistance  as  to  poor  contact  between 
particles  of  active  material.  The  electrolyte  can  get 
into  the  plate  or  the  grid  well  enough,  and  the  in- 
ternal resistance  of  the  cell  can  therefore  not  be 
very  high.  But  the  capacity  of  the  plate  has  suf- 
fered because  a  good  deal  of  what  ought  to  be  avail- 
able active  material  has  been  incapsulated  by  sulphate 
and  removed  from  the  reach  of  plate  activities. 

In  ordinary  practice,  the  cell  is  discharged  only 
until  its  electromotive  force  sinks  to  about  1.7  volts. 
This  means  that  only  perhaps  a  quarter  of  the  active 
material  of  the  plates  has  entered  into  reaction,  and 
that  the  increased  resistance  in  the  active  mass  is  due 
rather  to  separation  of  particles  by  sulphate  coatings 
than  to  complete  transformation  of  the  active  mate- 
rial at  any  place  into  insulating  material.  During 
the  charge,  sulphate  coatings  and  bridges  are  rapidly 
broken  down,  and  the  decrease  in  resistance  during 
charge  is  therefore  more  rapid  than  could  be  explained 
by  a  change  in  concentration  of  electrolyte  within 
the  pores  of  the  plate. 


INTERNAL  RESISTANCE  159 

After  a  period  of  discharge,  with  corresponding 
increase  in  resistance,  the  cell  recovers  its  original 
electromotive  force  along  a  carve  nearly  like  a 
diffusion  curve  when  the  circuit  is  opened.  It  also 
recovers  its  original  resistance  along  a  very  similar 
curve.  (See  Figure  65.)  This  fact  indicates  the 
dynamic  nature  of  the  equilibrium  which  causes  the 
cell  to  have  any  particular  electromotive  force  or  re- 
sistance at  a  particular  place  in  its  discharge,  charge, 
or  recovery  curve.  The  particles  of  active  material 
cannot  have  been  completely  covered  by  insulating 
sulphate,  for  on  standing,  the  plate  returns  to  its 
original  condition  as  far  as  we  can  measure  it  by  an 
examination  of  either  electromotive  force  or  resistance. 

We  must  evidently  think  of  the  lead  sulphate  as 
swelling  up  and  almost  plugging  canals  which  lead 
to  unchanged  lead  and  lead  peroxide.  The  density 
of  the  sulphate  is  much  less  than  that  of  the  materials 
from  which  it  is  formed,  and  while  the  particles  of 
lead  or  peroxide  may  have  had  plenty  of  space  be- 
tween- them  at  the  end  of  charge,  the  sulphate  must 
shut  off  much  of  this  from  activity  at  anything  like 
a  practical  rate  of  discharge.  As  long  as  no  current 
is  flowing,  acid  does  make  contact  with  the  remanent 
active  material  and  the  active  plane  in  the  plate 
draws  out  toward  the  exterior. 

In  Figure  62  the  full-line  curve  gives  the  open 
circuit  resistance  of  a  small  Plante  cell  at  various 


160  STORAGE  BATTERIES 

temperatures.  The  dotted  curve  shows  only  the 
shape  of  the  curve  for  the  electrolyte,  and  not  its  true 
value,  which  would  be  only  about  half  that  of  the 
cell  at  any  point.  The  acid  curve  was  plotted  in  this 
way  to  show  how  the  cell  resistance  departs  from  the 
acid  resistance  at  higher  temperatures.  Probably 
the  solid  resistances  of  grid  and  active  material  be- 
gin to  make  themselves  felt,  and  as  these  have  posi- 
tive temperature  coefficients,  the  increased  resistance 
makes  the  cell  take  a  sharper  turn  than  the  elec- 
trolyte. That  the  resistance  of  the  plate  material 
becomes  a  factor  is  shown  by  the  fact  that  pasted 
plates  of  slightly  greater  area,  placed  as  nearly  as 
possible  the  same  distance  apart,  show  a  decidedly 
greater  resistance  on  open  circuit  than  the  Plante 
plates.  The  cells  with  paste  plates  have  about  25% 
higher  resistance. 

97.  Effect  of  Distribution  of  Material  on  Resistance 
Curves.  —  The  curves  of  Figure  49  speak  for  them- 
selves. The  only  queer  thing  about  them  is  the  flat 
place  which  appears  after  60  to  80  minutes  of  discharge. 
This  is  characteristic  of  Plante  plates  with  ribs,  and 
does  not  appear  in  the  curves  for  paste  plates.  The 
ribs  of  these  plates  are  formed  into  active  material, 
which  lies  close  to  the  ribs  at  their  tops,  but  which 
forms  a  solid  mass  down  at  the  bottoms  of  the  ribs. 
(See  Figure  67.)  During  the  first  part  of  the  dis- 
charge the  electrolyte  finds  active  material  on  the 


INTERNAL  RESISTANCE 


161 


ribs,  and  diffusion  takes  place  largely  through  the 
open  space  between  them,  and  only  for  a  small  dis- 
tance through  active  material.  As  this  easily  avail- 
able material  is  used  up,  the  action  moves  farther 
down  into  the  plate  and  presently 
reaches  the  mass  of  material  at  the 
bottom  of  the  grooves.  Here  for 
a  time  there  is  material  enough  at 
a  nearly  constant  distance  from  the 
surface  of  the  plate,  and  after  this 
has  been  passed  the  resistance  rises 
very  rapidly  and  the  plate  poten- 
tial shows  that  the  cell  is  com- 
pletely discharged. 

If  there  is  anything  in  our  fun- 
damental theory  of  the  dependence 
of  electromotive  force  on  acid  con- 
centration, the  curves  of  electro- 
motive force  of  these  Cells  OUght  FIG.  67.  —  Diagram  of 
.  distribution  of  active 

to  sllOW  a  Corresponding  flat  place        material    on    ribbed 

somewhere  near  the  same  point  in  Plant6  Plate- 
the  discharge  curve.  The  curves  of  Figure  52  show 
it  clearly  except  in  the  one  for  8°  C.  We  missed  it 
here  by  not  taking  points  near  enough  together,  for 
it  shows  clearly  in  the  curve  of  Figure  64,  which  was 
made  on  the  same  cell  at  another  time.  This  curve 
gives  the  course  of  electromotive  force  and  resistance 
during  a  complete  discharge  followed  by  partial  re- 


162 


STORAGE  BATTERIES 


versal.  If  our  explanation  is  correct,  the  resistance 
ought  to  decrease  very  rapidly  after  passing  through 
a  maximum  at  about  the  end  of  complete  discharge. 
The  curve  is  in  agreement  with  this  idea. 


7 


X)9 


.08 


J07 


15' 


25° 


J05 


60 


80 


160 


TIME- MINUTES 


FIG.  68.  —  Change  of  internal  resistance  during  discharge  at  various 
temperatures.    Paste  plates. 

98.  Paste  plates  show  smooth  curves  of  resistance, 
as  shown  in  Figure  68. 

Our  resistance  curves  should  also  be  characteristic 
when  taken  for  different  rates,  and  Figure  69  shows 
this  for  the  same  Plante  plate  cell  at  constant  tem- 
perature. 

99.  A  most  interesting  idea  of  the  lively  dynamic 


INTERNAL  RESISTANCE 


163 


nature  of  the  momentary  equilibrium  existing  in  the 
cell  at  any  time  during  the  cycle  is  obtained  by 
plotting  curves  of  constant  composition  at  various 
times  and  temperatures.  The  curves  of  Figure  63 


/ 

^ 

4c 

y 

J 

&~ 

£ 

Z- 

•r-  ~ 

^ 

————— 

7Q* 

08 

"AMPEF 

r*" 

20               40               60               80               100               120             (40 
TlldE-MINUTES 

FIG.  69.  —  Change  of  internal  resistance  at  various  discharge  rates. 
Plante  cell. 

are  isothermal  curves.  Each  one  shows  the  course 
of  the  change  of  resistance  during  discharge  at  con- 
stant rate  and  constant  temperature.  Since  Fara- 
day's law  is  true,  the  cell  contains  exactly  the  same 
amount  of  lead,  lead  peroxide,  lead  sulphate,  sul- 
phuric acid,  and  water  at  the  end  of  the  same  time 
of  discharge.  Curves  of  constant  composition  will 


164 


STORAGE  BATTERIES 


therefore  result  if  we  cut  these  isothermal  curves  at 
times  30  min.,  1  hr.,  2  hr.,  etc.,  and  plot  the  values 
so  found  —  resistance  against  temperature.  Figure 
70  shows  a  set  of  curves  so  found.  The  curve  T  =  0 
is  for  open  circuit,  and  it  gives  the  temperature 


g 

z.« 


<?x, 


£6 


J04 


£0° 


TEMPERATURE 


FIG.  70.  —  Resistance  curves  corresponding  each  to  constant  composi- 
tion of  plates  and  electrolyte  made  by  cutting  the  curves  of  Figure 
63  at  various  times. 

[For  example,  after  60  minutes  of  discharge  at  25°  C,  the  cell  had 
a  resistance  of  0.06  ohm.] 

resistance  curve  for  the  cell,  like  the  full  curve  of 
Figure  62,  but  on  a  different  scale. 

The  slope  of  the  curve  T=0  gives  the  tempera- 
ture coefficient  of  resistance  on  open  circuit  at  the 
temperature  corresponding  to  the  point  where  the 
slope  is  determined.  The  slope  at  any  point  on  one 


INTERNAL  RESISTANCE  165 

of  the  other  curves  is  the  temperature  coefficient 
corresponding  to  the  temperature  where  the  slope  is 
taken.  For  all  the  curves  except  ^=0  the  condi- 
tion of  the  cell  is  one  of  momentary  dynamic  equilib- 
rium. The  materials  are  in  the  cell,  without  any 
doubt,  but  their  distribution  depends  to  a  great  ex- 
tent on  the  temperature  at  which  discharge  has 
taken  place. 

100.  Temperature  Coefficient  during  Activity.  — The 
open  circuit  temperature  coefficient  is  about  1.5  % 
per  degree.     The  coefficient  after  150  min.  of  dis- 
charge is  23  %  per  degree.     This  latter  value  is  of 
course  not  like  an  ordinary  temperature  coefficient, 
but  it  is  most  expressive  of  the  lively  nature  of  the 
factors  which  determine  the  condition  of  a  lead  stor- 
age cell  at  any  moment  in  its  life. 

Corresponding  curves  for  cell  voltage  are  given  in 
Figure  71. 

101.  Capacity  and  Acid  Density.  —  At  this  point  we 
are  prepared  to  examine  the  question  left  unanswered 
on  page  137.     Why  does  the  capacity  of  our  cell 
reach  a  maximum  for  acid  of  density  about  1.22,  as 
appears  from  the  measurements  ? 

The  statement  requires  elaboration.  It  is  not  true 
at  all  if  the  cell  is  examined  at  various  working 
rates,  and  if  we  measure  merely  the  acid  density  in 
the  main  body  of  the  cell.  It  may  very  well  be  the 
truth,  if  we  take  into  account  the  dilution  of  the 


166 


STOEAGE  BATTERIES 


acid  in  the  pores  of  the  active  material,  and  if  we 
base  our  calculation  on  the  density  of  acid  inside 
the  plate. 

The  curves  of  Figure  53  show  how  the  capacity  of 
the  cell  changes  with  the  acid  concentration  in  the 


5  ' 


TEMPERATURE 


FIG.  71.  —  Voltage  curves  corresponding  to  constant  composition  of 
plates  and  electrolyte. 

[For  example,  after  60  minutes  of  discharge  at  25°  C,  the  cell  showed 
a  voltage  of  1.69.] 

electrolyte.  This  particular  set  of  curves  was  made 
with  paste  plates,  and  corresponding  curves  for  large 
surface  Plante  plates  would  show  some  difference 
in  shape  and  would  have  their  maxima  at  other 
points.  But  it  is  very  evident  in  every  case  that 
the  maximum  of  capacity  shifts  toward  the  region  of 
higher  acid  density  as  the  rate  is  raised.  Rate  must 
evidently  be  taken  into  account  in  making  any  state- 


INTERNAL  RESISTANCE 


167 


ment  about  the  relation  between  capacity  and  acid 
density.  This  becomes  still  more  evident  if  we 
examine  into  the  change  of  capacity  of  positive  and 
negative  plates  separately.  Figure  72  gives  data 
for  the  positive  plate  and  Figure  73  for  the  negative. 


10" 


80°  25" 

ACID  DENSITY 


FIG.  72.  —  Change  in  capacity  with  variation  of  acid  density.  At  dis- 
charge rates  of  1,  2,  4,  and  8  amperes.  Paste  positive  plates, 
measured  against  auxiliary  electrode. 

It  is  evident,  that  as  far  as  the  positive  plate  is 
concerned  we  must  go  up  to  a  very  high  value  of 
acid  density  to  reach  the  maximum  of  capacity, 
while  for  negatives  at  ordinary  rates  we  need  only 
acid  of  ordinary  density  to  bring  us  out  to  the  maxi- 
mum. For  the  positive  we  should  have  acid  of 


168 


STORAGE  BATTERIES 


density  1.32  and  higher.  For  the  negative  we  need 
only  to  go  as  far  as  1.2,  which  is  well  within  the 
range  of  practical  operation.  The  facts  have  some- 
what the  appearance  of  contradicting  the  explana- 


10° 


15° 


£0°  £5' 

ACID  DENSITY 


30° 


351 


FIG.  73.  —  Change  in  capacity  with  variation  in  acid  density  at  va- 
rious rates.    Negatives. 

tion  which  Dolazalek   gives   for   the  appearance  of 
this  maximum.     He  says :  — 

"  At  the  beginning  of  discharge  the  current  lines 
enter  only  the  outer  layers  of  the  active  material, 
where  they  find  the  least  resistance.  As  the  change 
in  concentration  develops  polarization  in  the  outer 
layers,  the  current  lines  penetrate  deeper  and  deeper 
into  the  plate,  and  these  lines  have  density  such  that 


INTERNAL  RESISTANCE  169 

everywhere  in  the  pores  the  drop  in  potential  (ir)  is 
equal  to  the  polarization  prevailing  in  the  outer 
layers.  This  condition  must  of  necessity  be  ful- 
filled, for  the  active  material,  lead  as  well  as  lead 
peroxide,  is  a  good  conductor,  and  the  potential  must 
therefore  be  the  same  in  the  pores  and  out  near  the 
surface  of  the  plate.  If  the  polarization  in  the  outer 
layers  has  reached  0.2  volt,  the  potential  of  the 
whole  accumulator  has  also  fallen  by  the  same 
amount,  and  this  would  be  the  point  at  which  dis- 
charge would  be  stopped.  At  this  time  the  current 
lines  have  penetrated  so  far  into  the  active  material 
that  the  drop  of  potential  in  the  pores  —  the  product 
of  current  and  pore  resistance  (ir)  —  has  also  reached 
the  value  0.2  volt. 

"  But  the  resistance  of  the  pores  is  determined  by 
the  conductivity  of  the  acid  which  fills  them.  The 
better  the  acid  conducts,  the  later  the  moment  will 
appear  when  the  product  (ir)  reaches  the  value  0.2 
volt,  and  therefore  the  greater  the  capacity  of  the 
cell.  The  conductivity  of  sulphuric  solution  increases 
at  first  with  increase  of  concentration,  reaches  a 
maximum  at  30%  of  H2SO4,  and  then  decreases 
again.  The  above  discussion  shows  that  the  capacity 
must  also  reach  its  maximum  for  30%  acid,  and  this 
is  splendidly  confirmed  by  the  measurements." 

As  a  matter  of  fact,  it  is  quite  evident  from  the 
curves  that  the  measurements  do  not  confirm  this 


170  STORAGE  BATTERIES 

conclusion  at  all,  if  we  confine  our  measurement  of 
acid  density  to  reading  a  hydrometer  placed  in  the 
cell  electrolyte  between  the  plates.  But  if  we  con- 
sider at  the  same  time  the  difference  between  the 
density  of  the  acid  in  the  pores  and  that  in  the  main 
body  of  the  electrolyte,  —  this  same  difference  which 
we  have  already  had  occasion  to  mention  so  often,  — 
Dolazalek's  hypothesis  fits  much  better. 

102.  The  Concentration  of  the  Active  Ion.  —  The 
ion  which  really  determines  the  electromotive  force 
at  the  cathode  (the  peroxide  plate  during  discharge) 
is  H+,  and  the  current  is  driving  this  ion  toward  the 
peroxide,  2  H+  for  each  SO4  sent  in  the  opposite 
direction,  and  five  times  more  rapidly  as  well,  because 
of  its  greater  migration  velocity.  The  reaction  at 
this  electrode  requires  4  H+  for  each  PbO2,  and  2  H2O 
is  formed  as  the  result  of  the  reaction.  Besides  the 
lowering  of  acid  density  due  to  the  formation  and 
precipitation  of  PbSO4,  we  are  diluting  our  elec- 
trolyte by  the  addition  of  2  H2O.  Acid  of  maximum 
conductivity  is  about  1  of  H2SO4  to  19  of  H2O, 
and  it  may  very  well  be  possible  that  the  acid  con- 
centration out  in  the  cell  is  much  higher  than  it  is  in 
the  pores  at  the  place  where  the  reaction  is  taking 
place. 

At  the  negative  plate  (the  lead  plate)  things  are 
not  so  bad.  Here  SO4 —  is  the  determining  ion,  and 
it  is  used  up  in  the  pores  to  form  PbSO4,  more  being 


INTERNAL  RESISTANCE  171 

sent  along  as  an  ion  by  the  current.  Here  we  do  not 
have  the  formation  of  water  to  dilute  the  acid  at  the 
point  of  reaction,  and  in  spite  of  the  fact  that  the 
SO4  moves  much  more  slowly  than  H+,  there  is  less 
change  of  density  inside  the  plate.  A  smaller  excess 
density  in  the  main  body  of  the  electrolyte  is  sufficient 
to  maintain  the  concentration  at  the  point  of  action. 


CHAPTER  XIII 

PHYSICAL  CHARACTERISTICS 

103.  So  far  we  have  been  considering  the  chemical 
processes  in  the  cell  and  the  behavior  of  the  elements 
of  the  cell  under  varying  conditions.  We  have  not 
paid  much  attention  to  the  physical  nature  of  the 
plates  and  we  have  been  judging  them  by  their 
works  rather  than  by  their  looks.  It  is  interesting 
to  examine  the  plates  of  our  cell  somewhat  more 
closely  —  they  sometimes  give  a  good  deal  of  valu- 
able information. 

Most  of  the  hard  battery  service  is  done  by  plates 
of  the  Plante  type.  This  name  does  not  now  mean 
the  lead  sheets  used  by  the  inventor,  but  indicates 
that  the  active  material  of  the  positive  plate  has  been 
formed  from  metallic  lead  and  not  from  a  paste  of 
lead  salts.  For  the  hard  service  into  which  these 
plates  are  called  certain  fundamental  properties  are 
necessary.  Most  important  of  all  is  the  power  to 
deliver  current  at  a  high  rate  with  a  reasonable 
efficiency.  A  reasonable  life  must  also  be  given 
under  these  service  conditions. 

The  type  is  a  comparatively  simple  one.  It  may 
172 


PHYSICAL   CHARACTERISTICS  173 

be  represented  diagrammatically  by  Figure  66.  Its 
special  characteristics  are  : — 

Large  surface. 

Active  material  near  conducting  plate  and  elec- 
trolyte. 

Reserve  of  metallic  lead  for  further  formation  in 
service. 

A  certain  minimum  of  mechanical  strength. 

A  new  positive  plate  of  this  type  should  have  just 
enough  peroxide  on  it  to  give  its  rated  capacity, 
without  much  to  spare.  This  peroxide  has  been 
formed  in  the  factory  under  the  most  favorable  con- 
ditions, and  it  may  even  contain  a  little  cement  sul- 
phate from  its  rapid  formation.  If  it  goes  into  good 
hard  service,  it  probably  loses  a  full  quarter  of  this 
original  peroxide  in  a  few  months.  Whatever  there 
was  on  the  plate  that  was  at  all  loose  or  liable  to  be- 
come so,  has  been  knocked  off  by  the  rapid  evolution 
of  gas  during  charge.  By  this  time  the  original 
material,  whatever  its  nature  may  have  been,  has 
been  replaced  by  a  firmly  adherent  and  dense  layer 
of  peroxide  which  hugs  close  to  the  lead  of  the  plate. 
Ribs,  rosettes,  and  pores  have  opened  to  better  diffu- 
sion of  the  electrolyte,  and  the  plate  with  its  rather 
"  skimpy  "  but  readily  accessible  peroxide  layer,  is  in 
the  pink  of  condition  for  hard  work.  Its  capacity  at 
the  high  rate  at  which  it  is  working  is  perhaps  even 
increased,  in  spite  of  the  fact  that  it  has  lost  a  good 


174  STORAGE  BATTERIES 

quarter  of  the  active  material  with  which  it  started 
to  work  and  has  probably  regained  but  a  very  small 
fraction  of  the  loss. 

If  this  same  positive  plate  has  gone  into  slow  and 
easy  service,  it  will  also  change,  though  not  so  much 
in  external  appearance,  after  a  few  months  of  service. 
Its  ribs  or  rosettes  will  become  filled  with  peroxide, 
and  it  will  increase  in  total  capacity.  Too  low  a 
charge  rate  is  liable  to  crowd  the  spaces  in  the  plate 
and  produce  buckling  or  twisting. 

In  either  case  the  plate  seems  to  adapt  itself  as 
well  as  it  can  to  existing  conditions  —  to  its  "envi- 
ronment." It  increases  its  capacity  at  the  rate  at 
which  it  is  called  upon  to  work.  If  now  the  high 
and  low  rate  plates  were  to  be  interchanged,  the  one 
going  into  easy  service  instead  of  hard,  and  vice  versa, 
there  might  be  trouble  for  a  while.  The  "  skimpy  " 
skin  formation,  which  was  just  what  was  needed  at 
the  high  rate,  will  not  give  the  low  rate  capacity 
which  the  other  plate  has  been  easily  delivering. 
And  the  low  rate  plate  will  nearly  explode  when  it  is 
first  put  on  at  the  high  rate.  It  throws  off  excess 
active  material  for  a  time  and  as  remanent  sulphate, 
always  present  in  a  plate  worked  at  very  low  rates, 
is  cleared  away,  action  on  the  support  plate  itself  may 
be  severe  for  a  time.  Buckling  or  stretching  may 
appear.  If  the  plate  passes  this  danger  point  safely, 
it  settles  down  to  the  high  rate  pace  and  becomes 


PHYSICAL   CHAEACTEE1STICS  175 

before  "very  long  much  like  its  predecessor.  In  the 
meantime  the  former  high  rater,  which  made  so  poor 
a  showing  during  its  first  few  cycles  at  the  low  rate, 
has  picked  up  gradually.  More  material  forms  from 
the  reserve  lead  under  the  low  charge  rate,  and  most 
of  this  remains  in  the  plate.  Gradually  the  capacity 
rises  until  it  is  quite  sufficient  for  the  work,  and  by 
this  time  the  two  plates  have  completely  interchanged 
their  natures  and  looks.  It  seems  to  be  generally 
true  that  a  plate  that  has  been  working  at  high  rates 
is  in  no  special  danger  when  put  on  easier  work.  The 
reverse  is  not  true  by  any  means.  It  is  a  ticklish 
operation  to  break  in  a  plate  for  high  rate  work 
which  has  been  in  operation  for  a  long  time  in  very 
easy  service. 

104.  Densities.  —  If  we  examine  the  densities  and 
the  relative  volumes  occupied  by  lead,  lead  sulphate, 
and  lead  peroxide,  it  is  immediately  evident  that 
shrinking  and  expansion  are  sure  to  occur  during 
charge  and  discharge.  The  following  table  gives 
the  data :  — 

DENSITIES 

Metallic  Lead 11.4 

Peroxide,  hydrated 7.4 

Peroxide,  dry 9.4 

Lead  Sulphate 6.2 

Litharge ,    i    .    .  ...'•?.•  9.3 

Red  Lead  8.9 


176  STORAGE  BATTERIES 

A  good  many  pretty  mysterious  occurrences  in 
battery  practice  should  be  referred  directly  to  these 
differences  of  density.  For  instance,  most  Plante 
plates,  during  the  process  of  making  them  from  pure 
lead,  grow  in  length.  Some  of  those  with  long  ver- 
tical ribs  without  many  breaks  in  them  may  grow  an 
inch  in  length  per  foot  of  plate.  There  is  every  rea- 
son to  believe  that  this  stretching  is  caused  wholly 
by  the  crowding  of  sulphate  as  it  is  formed  from 
lead.  A  properly  forming  plate  has  its  sulphate  in 
the  form  of  a  very  dense  and  firmly  coherent  layer, 
and  as  this  is  formed  from  the  soft  lead  of  the  ribs 
it  hangs  to  them  and  crowds.  The  cumulative  effect 
is  proportional  to  the  length  of  unbroken  rib  along 
which  the  crowding  takes  place,  and  the  stretching 
is  proportional  to  this  factor  also.  It  is  also  very 
different  for  various  forming  agents,  probably  be- 
cause the  coherence  of  the  sulphate  to  the  lead  of 
the  plate  is  different  for  each. 

It  will  be  noticed  from  the  table  of  densities  that 
the  peroxide  layer  which  is  finally  formed  on  the 
positive  as  the  result  of  formation  is  denser  than  the 
sulphate  from  which  it  came.  So  the  properly  formed 
Plante  plate  has  a  peroxide  layer  with  just  about  the 
right  degree  of  porosity.  If  its  active  material  were 
more  porous,  it  would  be  at  the  expense  of  coherence  ; 
and  if  it  were  denser,  diffusion  would  be  poor,  and 
the  plate  would  give  low  capacity  at  high  rates. 


PHYSICAL   CHARACTERISTICS  177 

Metallic  lead  is  the  densest  of  the  materials  in  the 
table,  and  negative  plates,  which  are  to  be  porous, 
too,  if  they  are  to  have  reasonably  good  capacity, 
must  be  made  to  have  very  large  and  highly  devel- 
oped surfaces.  This  can  be  more  or  less  successfully 
attained  in  the  case  of  Plante  plates  by  the  natural 
method  of  forming  them.  True  Plante  negative 
plates  are  always  made  by  formation  first  as  perox- 
ide, by  attack  of  a  forming  agent  and  action  of  the 
current  on  the  pure  lead  of  the  grid.  They  are  sub- 
sequently completely  reversed  to  sponge  lead  and 
are  then  finished  negatives.  The  surface  is,  of 
course,  enormously  increased  by  the  formation  of 
grains  of  peroxide  from  the  solid  lead,  and  when  the 
reversal  is  given  to  the  negative  condition,  sponge 
lead  is  formed  right  where  the  grains  of  peroxide 
were.  Since  its  density  is  greater,  it  only  partially 
fills  the  space  occupied  by  the  particle  of  peroxide  or 
sulphate,  and  as  a  matter  of  fact  it  is  more  like  a 
mere  slender  network  when  the  plate  is  finished  than 
like  the  dense  solid  from  which  it  came. 

Paste  plates  make  natural  negatives.  Litharge 
and  red  lead  are  dense  compared  with  sulphate,  and 
if  the  paste  plate  is  allowed  to  sulphate  as  completely 
as  possible  before  formation  and  is  then  reduced  to 
lead,  the  resulting  sponge  has  passed  through  the 
state  of  lead  sulphate,  with  its  greater  volume,  and 
has  then  gone  on  to  become  metallic  lead,  shrinking 


178  STORAGE  BATTERIES 

all  the  time  during  this  latter  change,  and  opening 
pores  everywhere  during  the  final  change. 

The  extremely  small  solubility  of  lead  peroxide 
probably  accounts  for  the  fact  that  it  is  always  pres- 
ent in  fine  grains,  which  never  grow  to  any  size,  even 
after  many  cycles  of  service.  It  cannot  stay  in  solu- 
tion long  enough  to  move  about  and  look  for  a  place 
to  settle  where  there  is  already  a  crystal  of  peroxide. 
Lead  sulphate  is  comparatively  soluble,  and  when 
metallic  lead  is  formed  from  it,  the  lead  ion  has  a 
chance  to  look  for  a  nucleus  of  lead  on  which  to 
precipitate.  The  result  is  that  negative  plates  in- 
crease in  average  size  of  grain  with  service,  and  finally 
show  a  much  decreased  capacity  as  compared  with 
their  original  one.  Not  because  there  is  less  lead  in 
the  plate,  but  because  the  available  surface  has  be- 
come smaller. 


CHAPTER   XIV 

FORMATION  OF  PLANTE  PLATES 

105.  In  the  early  days  of  lead  storage  cells,  forma- 
tion was  a  very  slow  and  expensive  process,  requiring 
a  month  or  more  for  its  completion  and  the  expendi- 
ture of  a  great  deal  of  primary  battery  material.  For 
at  that  time  the  primary  cells  were  the  only  source 
of  current  for  the  purpose,  and  primary  cells  have 
never  been  very  cheap  as  a  source  of  power.  The 
plates  of  those  early  batteries  were  really  plates  of 
lead,  either  quite  flat  or  with  slight  corrugations 
which  enabled  them  to  hold  a  little  more  active 
material  on  the  roughened  surfaces.  These  plates 
were  set  up  in  their  final  cell  positions  in  dilute  sul- 
phuric acid,  usually  in  acid  much  more  dilute  than 
we  now  use  for  the  purpose.  The  cells  were  then 
subjected  to  a  series  of  reversals  —  they  were  charged 
first  in  one  direction  and  then  in  the  other. 

When  the  acid  is  poured  into  the  cells,  thin  layers 
of  lead  sulphate  form  on  both  plates,  and  this  process 
ceases  as  soon  as  the  layer  has  become  thick  enough 
to  protect  the  plate  from  further  action.  Charge  is 
begun  in  either  direction,  as  the  plates  are  just  alike 

179 


180  STORAGE  BATTERIES 

and  there  is  no  reason  to  decide,  at  this  point,  which 
plate  is  eventually  to  become  peroxide  and  which  is 
to  become  sponge  lead.  Under  the  action  of  the 
current  the  lead  sulphate  layer  at  the  anode  is 
changed  into  peroxide  and  that  at  the  cathode  is 
changed  to  sponge  lead.  The  thin  peroxide  layer  is 
then  a  complete  protection  against  further  action  and 
the  other  plate  is  cathode,  and  needs  no  protection. 
As  soon  as  charge  has  been  carried  this  far,  the  cell 
becomes  a  gas  generator  and  nothing  more.  All  the 
current  is  used  to  produce  hydrogen  at  the  cathode 
and  oxygen  at  the  anode. 

The  capacity  of  such  a  cell  is  very  small  indeed. 
It  will  give  a  spark  if  it  is  short-circuited,  but  not 
much  more.  For  the  amount  of  lead  sulphate  which 
is  formed  before  a  lead  plate  protects  itself  against 
further  action  by  the  acid  is  minute,  and  no  more 
sponge  lead  can  be  formed  at  the  negative  than  cor- 
responds to  the  original  quantity  of  sulphate  on  it. 
At  the  peroxide  plate  there  will  be  action  on  the  lead 
of  the  plate  and  formation  of  somewhat  more  sulphate 
than  was  originally  present,  but  this  action  takes 
place  only  during  a  part  of  the  charge,  and  before 
long  the  dense  peroxide  layer  shuts  off  the  lead  plate 
completely  from  further  attack. 

If  now  the  cell  be  immediately  put  on  charge  in 
the  opposite  direction,  the  results  are  not  good.  The 
active  material  formed  during  the  first  charge  turns 


FORMATION  OF  PL  ANTE  PLATES  181 

over  very  quickly  and  the  plates  reverse  their  po- 
larity, but  only  a  little  more  active  material  is  pro- 
duced. It  took  Plante  only  a  short  time  to  find  out 
that  much  better  results  were  obtained  by  letting  the 
cell  stand  discharged  before  each  reversal.  After 
standing  at  rest,  discharged,  for  a  day  or  so,  the  cell 
is  reversed.  Not  much  is  gained  in  the  way  of 
capacity  this  time,  but  when  the  cell  is  again  reversed 
it  is  found  that  considerable  gain  has  been  made. 
Local  action,  especially  at  the  peroxide  plate,  has  re- 
sulted in  deeper  attack  on  the  lead,  and  subsequent 
reversals  and  periods  of  rest  give  finally  an  active 
material  layer  of  useful  thickness. '  The  two  plates 
look  different  after  they  have  been  formed.  There 
is  a  layer  of  brown  peroxide  on  one  and  a  layer  of 
gray  sponge  lead  on  the  other. 

If  the  capacity  was  forced  too  far  by  more  forma- 
tion, the  peroxide  layer  was  liable  to  slough  off  and 
fall  to  the  bottom  of  the  cell.  To  be  sure,  more  was 
formed  to  take  its  place,  but  the  battery  has  reached 
its  maximum  capacity  and  further  formation  was 
merely  a  waste  of  current  —  an  expensive  article  in 
those  days. 

This  was  during  the  first  stage  in  development. 
Before  long  it  was  found  that  ribs  and  in  general 
mechanical  development  of  the  surface  of  the  lead 
plates  permitted  of  much  more  formation  and  so 
gave  higher  capacity.  Then  before  long  came  the 


182  STORAGE  BATTERIES 

idea  of  rapid  formation  —  the  use  of  chemical  agents 
to  aid  and  hasten  the  electrolysis,  and  along  these 
lines  the  modern  "  rapid  forming  processes  "  gradu- 
ally came  into  use.  There  are  many  points  about 
the  older  process  which  are  interesting  and  which 
lead  directly  to  an  explanation  of  the  theory  of  the 
later  methods  of  formation. 

The  first  point  to  be  remembered  is  that  lead  sul- 
phate does  not  form  a  dense  enough  layer  on  lead  to 
protect  it  from  the  action  of  an  electric  current  in 
sulphuric  acid.  A  plate  is  quite  protected  by  such 
a  layer,  provided  no  current  is  passing,  but  it  has  no 
power  to  resist  the  more  active  attack  of  the  anion, 
backed  by  the  driving  force  of  the  current. 

The  second  point  is  that  a  connected  layer  of 
peroxide  does  protect  against  attack,  even  when  the 
plate  is  anode  and  current  is  passing  through  the 
cell.  The  other  point  to  be  kept  in  mind  is  that 
the  positive  plate  can  discharge  itself  by  "  local 
action  "  while  it  is  at  rest.  In  the  case  of  the  Plante 
plate,  with  its  thin  coating  of  active  material,  this 
self -discharge  may  be  pretty  nearly  a  complete  one 
in  the  time  of  rest  recommended  for  Plarite  formation. 

The  curve  of  Figure  74  shows  how  rapidly  this 
action  takes  place  in  the  case  of  a  plate  which  has 
been  subjected  to  only  a  few  Plante  cycles,  and  which 
has  therefore  a  very  thin  layer  of  peroxide  on  its  sur- 
face. 


FORMATION  OF  PL  ANTE  PLATES 


183 


100 


\ 


80 


60 


It  is  the  most  natural  thing  in  the  world  that  such 
a  plate  should  discharge  itself  on  standing,  for  it  is 
really  a  whole  storage  cell.  Lead  plate,  peroxide 
plate,  sulphuric  acid,  all  are  present  in  every  per- 
oxide plate,  and  the  surface  of  contact  is  very  large 
in  proportion  to  the  mass  of  peroxide.  It  discharges 
during  its  period 
of  rest  wherever 
lead  and  peroxide 
are  in  contact,  and 
lead  sulphate  is 
formed  at  these 
points.  During 
the  subsequent 
reversal  all  the  ma- 
terial on  the  per- 
oxide plate  is  con- 
verted into  Sponge  FlG.  74.  _  Self-discharge  of  original  Plants 
lead,  and  this  in-  plate- 

eludes  new  sulphate  formed  from  the  plate  itself  as  a 
result  of  the  local  action  following  the  previous  per- 
oxidation.  During  the  rest  now  taking  place  after 
reversal  local  action  is  increasing  the  sulphate  con- 
tent of  the  other  (peroxide)  plate,  and  so  on. 

The  pertinent  query  arises :  Why  does  not  every 
peroxide  plate  discharge  itself  by  local  action  ?  It 
does,  but  only  to  the  same  extent  that  the  older 
Plante  plate  would.  Where  lead  and  lead  peroxide 


\ 


6       8 


10     12 
HOURS 


14     16     18    20 


184  STORAGE  BATTERIES 

are  in  contact  every  positive  plate  discharges  itself, 
but  the  amount  of  material  in  contact  in  a  modern 
plate  is  so  small  in  proportion  to  the  total  amount 
of  active  material  in  the  plate  that  the  amount  of 
action  on  the  plate  is  comparatively  small,  and  only 
a  low  percentage  of  the  total  capacity  of  the  cell  is 
lost  through  the  effect.  The  action  is,  however, 
strictly  proportional  to  the  surface  of  contact  be- 
tween lead  and  peroxide,  and  the  modern  high-rate 
plates  are  subject  to  much  greater  losses  from  this 
cause  than  are  the  paste  plates.  Fortunately  the 
efficient  large  surface  plate  does  its  important  work 
under  conditions  of  rapid  reversal  —  discharge  and 
charge  follow  each  other  very  rapidly,  and  the  cell 
is  never  standing  at  rest  for  more  than  a  few  min- 
utes at  a  time. 

106.  Modern  « Rapid  Plante "  Formation.  —  After 
the  first  excitement  over  Plante's  discovery  had 
passed,  it  was  not  very  long  before  the  small  ca- 
pacity of  the  flat  plates  was  felt  to  be  a  drawback. 
The  surface  was  increased  by  corrugating  or  other- 
wise roughening  it.  At  this  same  time  the  original 
method  of  forming  by  a  series  of  reversals  began  to 
seem  slow  and  wasteful  of  current.  So  methods 
were  sought  which  should  permit  of  attaining  the 
same  or  better  results  more  easily  and  rapidly,  and 
these  methods  were  :  — 

1.    To  begin  the  attack  on  the  lead  by  treating  the 


FORMATION  OF  PLANTS  PLATES  185 

plate  with  an  etching  agent,  nitric  acid,  for  example, 
which  dissolves  some  of  the  lead  and  roughens  the 
surface  of  the  plate.  This  treatment  was  followed 
by  regular  Plante  formation,  but  the  process*  went 
on  much  more  rapidly  than  in  the  original  method. 

2.  To  produce  on  the  surface  of   the  lead   plate 
some  compound  which  could  afterward  be  changed 
into  peroxide  by  a  single  charge.     One  of  these  ideas 
was  to  subject  the  plate  to  the  action  of  sulphur. 
Lead  sulphide  was  formed,  and  this  was  changed  first 
into  sulphate  and  then  to  peroxide  during  the  period 
of  charge. 

3.  To  add  to  the  sulphuric  acid  used  in  formation 
an  agent  which  should  attack  and  dissolve  the  lead 
of  the  plate.     This  resulted  in  formation,  first  of  a 
soluble  lead  salt,  then  of  sulphate  by  reaction  with 
the  sulphuric  acid  of  the  electrolyte,  and  finally  of 
peroxide  by  the  usual  effect  of  the  current. 

This  last  method  is  the  usual  one  nowadays,  and 
the  great  majority  of  all  Plante  plates  are  now 
formed  from  lead  plates  by  electrolysis  in  a  sulphuric 
acid  solution  containing  a  "forming  agent."  The 
most  efficient  method  of  applying  this  principle  seems 
to  be  to  use  as  agent  a  substance  which  can  furnish 
an  anion  capable  of  forming  a  soluble  lead  salt. 

The  common  soluble  lead  salts  are :  the  nitrate, 
acetate  (chloride),  chlorate,  perchlorate,  and  sulphite, 
and  these  are  (or  have  been)  all  used  for  the  purpose. 


186 


STOEAGE  BATTERIES 


It  is  not  our  business  to  examine  technical  recipes 
or  to  study  the  minutiae  of  manufacturing  processes. 
But  we  can  state  a  general  theory  of  formation  which 
will  be  found  applicable  to  all  the  different  processes. 

107.  Theory  of  Rapid 
Formation.  —  Figure  75 
gives  a  diagrammatic 
picture  of  the  different 
zones  and  stages  in  the 
formation  of  a  lead  plate. 
All  plates  are  formed  into 
peroxide  first,  if  they  fall 
into  this  class  at  all,  even 
if  they  are  eventually  to 
become  negatives;  so  this 
one  picture  covers  all  the 
cases. 

The  solution  contains 
sulphuric  acid  and  the 
forming  agent,  which  has 
as  an  ion  an  ion  which  can 
yield  a  soluble  salt  of 
lead.  The  charging  cur- 
rent started,  this  forming  ion  and  SO4  migrate 
toward  the  plate.  The  velocity  of  the  forming  ion 
may  apparently  be  either  greater  or  less  than  that  of 
the  SO4  ion  without  making  any  difference  in  the 
process.  At  any  rate,  we  will  suppose  that  the  two 


FIG.  75.  —  Diagram  to  show  mod- 
ern "rapid  Plant6"  formation. 


FORMATION  OF  PLANTS  PLATES  187 

ions  reach  the  plate  at  the  same  time.  A  layer  of 
soluble  lead  salt  in  solution  is  formed  at  once,  but 
this  lasts  only  an  instant.  SO4  is  there  and  lead 
sulphate  is  immediately  precipitated.  The  regular 
charging  reaction  then  comes  into  play  and  the  sul- 
phate is  transformed  into  peroxide.  In  the  mean- 
time the  forming  ion  has  been  freed,  and  it  bores  into 
the  plate  again  to  form  more  soluble  material,  which 
is  precipitated  by  SO4  ,  and  so  on. 

This  insures  formation,  but  the  relative  concen- 
trations of  the  two  active  ions  must  be  carefully 
balanced  if  it  is  to  proceed  far  enough  to  make  it  a 
practical  success.  If  there  is  too  little  forming  ion 
in  proportion  to  the  sulphate  ion,  sulphate  will  pre- 
cipitate as  a  dense  layer  clinging  closely  to  the  plate, 
and  peroxidation  follows  so  closely  that  the  plate 
soon  protects  itself.  If  there  is  too  much  forming 
ion  relative  to  the  SO4 —  ion,  an  actual  layer  of  solu- 
tion, containing  a  considerable  concentration  of  the 
soluble  lead  salt,  forms  between  the  plate  and  the 
layer  of  precipitated  sulphate.  The  sulphate  layer 
is  thus  kept  from  close  contact  with  the  plate  at  all 
points,  and  when  peroxide  forms,  the  whole  sheet  of 
active  material,  partly  sulphate  and  partly  peroxide, 
is  so  loosely  attached  that  it  flakes  off  at  the  least 
provocation,  leaving  the  plate  bare. 

The  formation  of  a  tough  and  coherent  peroxide 
demands  careful  attention  to  the  relative  concentra- 


188  STORAGE  BATTERIES 

tions  of  the  active  ions.  It  may  be  taken  as  a  general 
rule  that  there  is  no  one  acid  concentration  and  no 
one  forming  ion  concentration  that  produce  correct 
formation.  For  each  acid  concentration  there  will 
be,  however,  an  optimum  concentration  of  the  form- 
ing ion,  and  other  considerations  usually  make  it 
advisable  to  use  a  rather  low  acid  concentration  for 
the  forming  solution. 

Formation  to  a  practical  depth  usually  requires 
eight  or  ten  times  the  number  of  ampere-hours  after- 
ward to  be  required  of  the  plate  in  service.  This  is 
quite  natural,  for  as  we  have  seen  in  Chapter  X,  we 
use  in  service  only  about  10  to  30  %  of  the  total 
active  material  of  the  plate.  If  the  plate  is  an  old- 
fashioned  thin-layered  flat  Plante  plate,  the  maximum 
proportion  of  the  total  will  be  brought  into  use.  If 
it  is  a  modern  plate  with  ribs  or  rosettes,  a  smaller 
part  of  the  total  peroxide  will  be  turned  over  in 
practice. 

108.  Low  Voltage  Formation.  —  A  special  mode  of 
formation  has  been  invented  and  patented  by  Pollak, 
and  while  it  has  apparently  not  been  adopted  as  a 
manufacturing  method,  it  is  of  interest  as  an  example 
of  a  principle  we  have  frequently  applied.  Lead 
sulphate  cannot  protect  a  lead  plate  from  attack 
when  current  is  passing  and  the  plate  is  anode.  If 
we  can  prevent  the  formation  of  lead  peroxide  and 
continue  to  form  sulphate,  there  is  no  reason  why 


FORMATION  OF  PLANTS'  PLATES  189 

formation  without  any  special  agent  should  not  be 
carried  as  far  as  we  choose. 

Peroxide  is  not  formed  from  sulphate  except  at 
cell  voltages  higher  than  2  volts.  If  therefore  we 
send  current  through  the  cell  at  a  voltage  slightly 
lower  than  this  value,  only  sulphate  will  result,  and 
the  plate  will  continue  to  be  attacked.  This  con- 
dition of  things  is  best  attained  by  connecting  the 
lead  blank  which  is  to  be  formed  to  a  fully  charged 
peroxide  plate  of  capacity  sufficient  to  complete  for- 
mation. This  means  a  charged  peroxide  plate  of 
eight  or  ten  times  the  capacity  desired  for  the 
finished  plate  we  are  making.  When  enough  sul- 
phate has  been  produced  to  give  final  capacity,  the 
sulphate-formed  plate  is  taken  out  of  this  cell  and 
formed  to  peroxide  in  another  cell,  either  against 
negative  plates  or  flat  lead  dummies.  In  the  mean- 
time the  auxiliary  forming  positives  are  receiving  a 
new  charge  to  get  them  ready  for  the  next  forma- 
tion. There  seem  to  be  practical  reasons  why  this 
idea  has  not  been  generally  adopted.  Theoretically 
and  as  a  laboratory  experiment  it  works  quite  per- 
fectly. 

109.  Changes  in  the  Forming  Agent  during  Forma- 
tion.—  It  is  much  to  be  desired  that  the  activity  of 
the  forming  agent  should  cease  as  soon  as  the  plate 
is  brought  up  to  its  proper  capacity.  If  some  of 
this  dangerous  substance  remains  in  the  plate,  it  will 


190  STORAGE  BATTERIES 

continue  its  original  behavior  and  attack  the  lead 
of  the  peroxide  plate  during  each  period  of  charge. 
Of  course  this  attack  is  much  weakened  by  the  fact 
that  the  plate  is  completely  peroxidized  and  also  be- 
cause it  is  never  discharged  to  such  an  extent  that 
much  of  the  peroxide  in  contact  with  the  lead  sup- 
port is  changed  to  sulphate.  But  a  lead  cell  must 
have  a  life  of  several  years  and  must  go  through  a 
great  many  cycles  of  charge  and  discharge,  and  even 
a  small  amount  of  action  can  be  cumulatively  harm- 
ful. 

Some  of  the  forming  agents  mentioned  in  the  list 
are  only  too  ready  to  eliminate  themselves.  When 
chlorine  ion  is  used  either  as  hydrochloric  acid  or  as 
a  chloride,  chlorine  gas  is  evolved  nearly  quantita- 
tively at  the  anode,  and  the  forming  agent  must  be 
replaced  accordingly.  Chlorates  are  also  broken  up 
with  evolution  of  chlorine,  but  not  so  completely  as 
Cl~  ion.  Nitric  acid  is  reduced  at  the  cathode,  first 
to  nitrous  acid  and  finally  to  ammonium  sulphate. 
This  necessitates  renewal  during  formation  and  final 
saturation  of  the  electrolyte  with  ammonium  sul- 
phate. This  means  that  small  quantities  of  nitric 
acid,  left  in  a  plate  as  the  result  of  formation,  are 
perfectly  eliminated  from  the  cell  during  its  first 
few  cycles  of  active  operation. 

An  interesting  suggestion  is  that  of  Beckmann. 
Sulphur  dioxide  in  water  solution  forms  some  sul- 


FORMATION  OF  PLANTS  PLATES  191 

phurous  acid,  H2SO3,  and  this  gives  a  forming  ion 
SO3  ,  because  lead  sulphite  is  a  fairly  soluble  sub- 
stance. 

During  formation  this  ion  leads  the  attack  on  the 
lead  plate  as  described,  but  it  is  itself  oxidized  rather 
readily  to  SO4  ,  and  so  a  few  cycles  are  sufficient 
to  remove  completely  every  trace  of  extraneous  ion 
from  the  cell.  This  also  seems  rather  difficult  to 
apply  as  a  practical  forming  process,  as  SO2  is  not  a 
pleasant  substance  to  have  about  in  large  quantities. 

Acetate  ion,  C2H3O2~,  is  most  persistent  and  can 
cause  great  damage  if  any  of  it  is  left  in  the  plate 
after  formation.  Even  this  is  gradually  destroyed 
as  the  result  of  cell  activity. 

Perchlorate  ion,  C1O4~,  is  apparently  the  only  sub- 
stance in  the  list  which  is  perfectly  resistant  to  the 
effects  of  the  current.  It  is  therefore  the  most  effec- 
tive of  all  forming  agents,  as  it  does  not  need  to  be 
renewed  at  all  in  the  forming  tanks.  For  this  same 
reason  it  might  become  a  dangerous  factor  in  the 
cell  after  it  goes  into  service.  Fortunately  the 
limits  of  proportion  between  which  perchlorate  ion 
can  act  as  a  forming  ion  in  sulphuric  acid  solution 
are  narrow.  In  electrolyte  the  sulphuric  acid  con- 
centration is  comparatively  high,  and  the  little  rem- 
nant of  perchlorate  is  therefore  a  very  small  fraction 
indeed.  Under  these  conditions  it  hardly  has  any 
power  of  attacking  lead,  and  while  in  proper  propor- 


192  STORAGE  BATTERIES 

tions  it  is  perhaps  the  most  active  of  all  forming  re- 
agents, it  is  much  less  dangerous  than  many  of  the 
others  in  the  conditions  of  ordinary  service. 

110.  Plante  Negatives.  —  The  negative  Plante  plate 
is  made  in  exactly  the  same  way  as  the  positive.  It 
is  formed  as  a  positive,  with  the  aid  of  a  rapid  form- 
ing agent,  and  is  then  reversed  completely,  so  that  all 
the  peroxide  is  changed  to  sponge  lead  under  the 
action  of  the  current. 

Such  a  plate  has  all  the  good  qualities  of  the  large 
surface  positive,  especially  during  the  first  part  of  its 
life.  It  is  easily  reached  by  the  electrolyte  and  can 
give  large  discharges  without  danger.  Later  in  its 
life  it  loses  a  considerable  part  of  its  original  capacity 
because  of  increase  in  size  of  grain  and  loss  of 
porosity.  It  must  therefore  be  made  to  have  a  much 
larger  original  excess  capacity  than  the  positive, 
which  increases  its  capacity  by  local  action  and  slow 
formation  in  service.  Most  Plante  negatives  are 
made  to  give  nearly  100  %  excess  capacity  when  they 
go  into  service.  This  excess  is  rather  rapidly  lost 
during  the  first  six  months  or  so  of  service,  and  at 
the  end  of  the  first  year  the  plate  will  usually  show 
an  excess  of  only  about  25%. 

The  curves  of  Figure  76  show  how  light  Plante 
positives  and  negatives  change  in  capacity  in  service. 
The  curves  are  of  course  only  averages,  and  differ- 
ent types  would  show  somewhat  different  curves,  but 


FORMATION  OF  PLANTS  PLATES 


193 


these  can  safely  be  taken  as  representing  the  gen- 
eral course  of  events. 

Many  makers  use  pasted  negatives  entirely,  even 
in  batteries  which  are  to  be  called  on  for  the  hardest 


o 

r 

- 


8" 


60 


500 

NUMBER   OF  CYCLES 

FIG.  76.  —  Change  in  capacity  in  hard  service.    Light  Plante  plates. 

service.  Their  life  is  sufficient,  and  their  excess 
capacity  is  so  great  that  no  fear  need  be  entertained 
that  the  negatives  will  ever  limit  the  discharge  of  the 
cell. 


CHAPTER  XV 

PASTE  PLATES 

111.  It  was  Faure  who  first  conceived  the  idea  of 
producing  active  materials  for  accumulator  plates  by 
the  electrolysis  of  lead  compounds  instead  of  from 
the  lead  of  the  plate  itself,  and  he  began  the  evolu- 
tion of  what  are  called  paste  plates.  Faure  probably 
reasoned  somewhat  like  this :  Plante  produces  lead 
sponge  and  lead  peroxide  by  a  wearisome  and  ex- 
pensive attack  on  the  solid  lead.  It  would  certainly 
be  much  better  to  cover  a  lead  plate  with  a  mass 
which  can  then  be  easily  and  completely  converted 
into  lead  at  the  cathode  and  lead  peroxide  at  the 
anode,  and  such  a  plate  can  be  made  to  have  capacity 
enormously  greater  than  the  thin-skinned  plates  of 
Plante.  Some  triumphs  and  not  a  few  troubles  for 
many  people  began  just  at  this  point  in  the  history 
of  galvanic  cells.  As  we  now  know  very  well,  Faure's 
invention  was  not  able  to  push  Plante's  out  of  the 
field.  Each  of  the  two  types  of  plate  has  a  perfectly 
definite  place  and  service  of  its  own,  and  while  the 
two  types  appear  to  cross  into  each  other's  territory 
now  and  then,  there  is  always  some  very  definite 
reason  for  the  apparent  intrusion. 

194 


PASTE  PLATES  195 

The  process  of  making  a  paste  plate  is  a  very 
simple  one.  Perhaps  the  people  who  find  most 
difficulty  in  the  process  are  the  ones  who  have  to 
actually  manufacture  the  plates  for  the  market.  The 
difficulties  are  all  practical  ones  and  so  minute  and 
difficult  to  sort  out  and  describe  and  remedy  that  we 
can  only  hope  to  touch  the  more  evident  and  funda- 
mental ones. 

Suppose  it  is  desired  to  make  a  set  of  fairly  light 
plates  to  be  used  in  an  electric  automobile.     They 
must  have  good  capacity  per  unit 
of    weight,    mechanical    strength 
sufficient  to  withstand  the  jar  of 
road  service,  and  a  fairly  long  life     FlG' ?7'  ~  *ib.B °f 

*  positive  paste  plate. 

(say  250  to  300  cycles),  if  they 
are  to  compete  with  other  plates  already  on  the  mar- 
ket.    We  will  make  the  positive  plates  first. 

For  positives,  a  grid  which  can  hold  the  peroxide 
in  place  somewhat  is  usually  considered  best.  Lead 
peroxide  has  very  little  coherence  and  drops  off  the 
plate  surface  very  easily  unless  it  is  kept  in  some  way 
from  doing  so.  We  should  therefore  choose  a  grid 
of  the  general  form  shown  in  Figure  77,  having  ribs 
with  inward  dovetails  to  keep  the  material  in  the 
plate.  It  is  usual  to  cast  the  grids  of  6  to  10  %  anti- 
mony alloy.  This  gives  a  much  stiffer  grid  than 
pure  lead  and  prevents  attack  by  the  acid  of  the 
electrolyte.  Molds  we  will  assume  —  they  are  not 


196  STORAGE  BATTERIES 

within  the  province  of  our  discussion  —  and  we  will 
also  assume  that  we  have  a  supply  of  grids  ready 
cast.  The  next  thing  is  to  paste  them. 

Recipes  for  positive  pastes  are  legion.  A  very 
simple  one  which  can  be  made  to  give  good  results  is 
made  by  mixing  litharge  (PbO),  or  red  lead  (Pb3O4), 
or  a  mixture  of  the  two,  with  rather  dilute  sulphuric 
acid.  A  paste  is  made  of  the  constituents,  just 
thick  enough  to  permit  of  its  being  worked  into 
the  holes  and  hollows  of  the  grid.  If  then  a  plate 
so  pasted  is  set  in  the  air,  it  dries  and  at  the  same 
time  sulphates,  setting  to  a  hard  mass.  Better  re- 
sults are  obtained  by  soaking  the  freshly  pasted  plate 
in  dilute  sulphuric  acid  for  several  days.  During 
this  time  what  is  perhaps  the  most  important  thing 
in  the  whole  life  of  the  plate  takes  place.  It  cements. 

Lead  peroxide  is  a  powdery,  non-coherent  mass  at 
best,  and  a  plate  pasted  with  pure  peroxide  has  very 
little  mechanical  strength  compared  with  the  plate 
which  has  been  treated  in  the  way  just  described. 
But  lead  sulphate,  crystallizing  into  a  firm,  connected 
mass  all  through  the  interstices  between  the  grains  of 
oxide  and  peroxide,  can  become  a  most  useful  bind- 
ing material.  Just  a  word  about  what  we  mean  by 
the  general  term  cement. 

A  cement  sticks  things  together.  It  does  this  by 
first  of  all  penetrating,  as  a  liquid,  all  the  irregu- 
lar holes  and  crannies  and  spaces  between  the  solid 


PASTE  PLATES  197 

particles  to  be  held  together.  It  then  afterward 
hardens  to  a  solid  and  fills  all  these  irregular  spaces, 
thus  dovetailing  the  various  pieces  of  the  whole  mass 
into  a  single  piece.  The  resulting  solid  is  as  strong 
as  its  two  final  constituents — one  of  them  the  original 
solid  which  was  to  be  bound  together,  the  other  the 
new  solid  formed  by  the  hardening  of  the  cement. 

If  red  lead  is  used  in  the  paste,  the  following  reac- 
tion takes  place  partially  as  soon  as  the  acid  used  in 
mixing  has  a  chance  to  react :  — 

Pb304  +  2  H2S04  =  2  PbS04  +  Pb02  +  2  H2O. 

The  plate  therefore  contains  lead  peroxide,  red  lead, 
and  lead  sulphate,  as  soon  as  it  has  set  and  before 
formation  is  begun.  If  litharge  alone  has  been  used 
in  the  paste,  the  unformed  plate  contains  only  lead 
oxide  and  lead  sulphate.  The  lead  sulphate  reacts 
quickly,  and  within  a  few  minutes  or  at  most  a  few 
hours  after  the  plate  has  been  placed  in  the  cement- 
ing acid  bath,  the  sulphation  of  the  plate  is  quan- 
titatively complete.  But  the  second  and  equally 
important  step  —  the  locking  together  of  the  plate 
by  the  sulphate  —  takes  place  much  more  slowly.  It 
depends  on  the  recrystallization  of  lead  sulphate  and 
is  an  action  very  like  the  dreaded  "sulphation"  which 
is  so  often  the  cause  of  trouble  in  the  vehicle  batteries 
all  over  the  country.  The  fine  particles  of  sulphate  are 
more  soluble  than  the  larger  ones,  and  the  latter  grow 


198  STORAGE  BATTERIES 

at  the  expense  of  the  smaller  ones.  As  the  crystals 
grow  they  interlace  and  lock  themselves  together,  as 
growing  masses  of  crystals  always  do.  One  sul- 
phate crystal,  growing  out  from  between  grains  of 
oxide  or  peroxide,  touches  the  one  growing  out  from 
the  neighboring  opening  and  the  two  coalesce.  The 
result  of  this  crystalline  growth  and  interlocking  is 
the  cementing  of  the  plate.  It  becomes  hard,  sounds 
hard  when  it  is  struck,  can  be  used  as  a  hammer  and 
pounded  on  the  floor  without  losing  any  paste  ex- 
cept at  the  place  where  the  lead  grid  is  actually  bent 
or  broken.  It  is  now  ready  to  be  formed. 

i"J3  Formation  of  Paste  Positives.  —  The  plate,  des- 
tined to  become  a  positive,  is  now  hung  in  a  bath  of 
rather  dilute  sulphuric  acid  and  made  the  anode  for 
the  passage  of  the  forming  current  for  perhaps  60 
hours.  Figure  78  shows  the  changes  which  take  place 
in  its  composition  during  this  time.  At  the  start 
the  plate  contained  :  — 

PbO  55  % 
Pb02  25% 
PbS04  20  % 

The  lead  oxide  begins  to  turn  to  peroxide  right 
away  as  soon  as  charge  is  begun,  but  the  sulphate 
content  of  the  plate  rises  for  several  hours.  This 
may  be  because  the  plate  is  becoming  more  porous 
as  formation  proceeds,  so  that  the  acid  finds  unused 


PASTE  PLATES 


199 


oxide  ready  to  hand  as  it  enters  new  channels.     But 
before  long  the  sulphate  also  passes  over  into  peroxide 


100 


a 

§eo 

Id 

1 

£  40 


20 


40 


£40 


60  IZO  160  200 

AMPERE-HOURS  FORMATION 

FIG.  78.  —  Changes  in  composition  of  a  paste  positive  during  formation. 

and  at  the  end  of  the  period  of  formation  the  active 
material  consists  of :  — 


PbO 
PbO, 


9% 
88% 


PbSO4    3% 

Our  cement  is  nearly  gone.  But  even  this  3% 
is  a  potent  factor  in  the  life  of  this  positive  plate, 
and  if  formation  has  been  carried  on  at  the  right 
current  density,  there  is  also  some  cementing,  or 
rather  loose  interlocking,  of  the  particles  of  peroxide. 

It  seems  probable  that  this  remanent  lead  sulphate 


200  STORAGE  BATTERIES 

is  never  removed  from  the  plate  under  proper  condi- 
tions of  charge  and  discharge  and  that  it  forms  a  net- 
work which  really  helps  to  hold  the  peroxide  together. 
During  each  discharge  sulphate  is  deposited  on  this 
nucleus,  and  the  plate  may  perhaps  be  partially  held 
together  by  the  binding  action  so  produced  during 
the  succeeding  period  of  charge,  which  is  so  trying 
to  the  paste  plate. 

Surety  this  cannot  be  the  whole  story  of  the  making 
of  a  paste  positive  ?  There  are  hundreds  of  secrets 
carefully  guarded,  and  hundreds  of  patents  and  reci- 
pes for  pastes.  A  glance  at  the  patent  literature 
shows  the  nature  of  the  various  things  that  might  be 
added  to  the  positive  paste  —  alcohols  and  organic 
acids,  salts  and  sugars,  and  almost  anything  else  that 
one  could  think  of.  The  intention  of  these  additions 
is  to  aid  in  producing  either  one  of  two  desirable 
things  :  — 

(a)  An  increase  in  the  hardness  of  the  plate,  and 
therefore  increased  life. 

(5)  An  increase  in  porosity,  and  therefore  its 
efficiency. 

The  organic  acids  —  carbolic  acid,  for  example 
—  hasten  the  cementing  action.  Probably  a  lead 
phenolate  or  some  such  substance  is  formed  and  lead 
sulphate  is  then  rapidly  produced  from  this.  The 
soluble  lead  salt  would  naturally  hasten  sulphation 
just  as  a  forming  agent  hastened  it  in  the  case  of 


PASTE  PLATES  201 

Plante  plates.  The  addition  to  the  paste  of  a  soluble 
salt  like  magnesium  sulphate  has  not  much  effect 
unless  the  plate  is  allowed  to  dry  after  pasting  and 
before  formation.  The  salt  crystallizes  all  through 
the  plate  while  it  is  dr}dng  and  setting,  and  is  then 
dissolved  again  during  formation,  leaving  spaces  in 
the  formed  active  material  and  thus  increasing  po- 
rosity. A  good  many  manufacturers  probably  still 
feel  the  need  of  a  "  hardening  agent "  or  a  "  porosity 
agent,"  or  both.  But  it  seems  perfectly  possible  to 
get  along  without  either  of  them.  And  perhaps  the 
final  result  is  just  about  as  satisfactory  if  only  lead 
oxide  and  sulphuric  acid  are  used  instead  of  the  more 
mysterious  and  cabalistic  formulae  of  some  of  the  in- 
ventors in  this  field.  It  is,  as  a  matter  of  fact,  very 
hard  to  see  how  any  good  effect  of  the  addition  of 
any  of  these  agents  to  the  paste  can  remain  after  the 
resulting  plate  has  been  through  fifty  cycles  of  hard 
work.  Long  before  that  time  the  hardening  agent 
has  been  completely  decomposed  and  removed  from 
the  cell  so  completely  that  chemical  analysis  will 
often  fail  to  show  a  trace  of  it.  The  porosity  agent 
is  of  course  dissolved  out  and  diluted  through  the 
cell  as  a  part  of  its  activity.  The  active  material  of 
the  plate  has  been  turned  over  and  over  and  has  dis- 
posed itself  in  new  ways  —  filling  up  the  old  pores 
and  channels  and  making  new  ones  for  itself.  All 
that  is  left  is  a  very  small  trace  of  lead  oxide  and  the 


202  STORAGE  BATTERIES 

normal  proportion  of  lead  peroxide  and  lead  sulphate. 
Whatever  coherence  the  paste  now  has  is  due  to  these 
two  substances,  and  as  we  have  already  seen,  lead 
peroxide  is  not  inclined  to  bind  together  to  give 
much  mechanical  strength.  The  remanent  network 
of  sulphate  is  all  that  holds  the  plate  together. 
Whenever  particles  of  peroxide  lose  contact  at  the 
surface  of  the  plate  their  fate  is  to  fall  off  sooner  or 
later  and  collect  in  the  bottom  of  the  containing  jar. 
The  cementing  sulphate  has  no  chance  to  persist  at 
the  surface.  It  is  transformed  almost  completely 
into  peroxide  at  each  charge.  So  the  peroxide  plate 
naturally  loses  active  material  by  "  shedding,"  and 
the  rapid  evolution  of  gas  which  accompanies  the  end 
of  each  charge  helps  to  throw  off  all  the  loose  par- 
ticles. It  is  the  fate  of  all  paste  positives,  even  the 
most  healthy,  to  finally  become  a  mere  skeleton  —  a 
grid  —  with  nothing  left  on  it  but  a  few  bunches  of 
peroxide  clinging  to  its  ribs. 

113.  Paste  Recipes.  —  Every  manufacturer  has  his 
own  particular  recipe  for  positive  paste.  This  and 
other  facts  lead  to  the  conclusion  that  the  propor- 
tions are  not  of  great  importance.  Many  manufac- 
turers make  good  plates,  and  they  use  — 

1.  Pure  litharge. 

2.  Pure  red  lead. 

3.  Mixtures  of  litharge  and  red  lead  in  all  propor- 
tions. 


PASTE  PLATES  203 

Some  makers  mix  their  paste  with  strong  sul- 
phuric acid ;  some  use  it  weak.  Evidently  there  is 
much  in  knowing  how  to  paste,  dry,  cement,  and 
form  —  much  more  than  in  any  secret  of  proportions 
or  materials. 

This  statement  might  almost  be  taken  as  an  axiom 
in  battery  manufacture. 

114.  Paste  Negatives.  —  The  finished  negative  paste 
plate  has  a  very  different  set  of  characteristics  and  a 
very  different  life  history  from  its  weaker  positive 
brother,  but  it  begins  in  very  much  the  same  way. 
Since  it  is  to  become  spongy  metallic  lead,  it  may  as 
well  be  made  of  litharge  unless  there  is  some  special 
reason  against  this,  for  the  step  from  PbO  to  Pb  is 
the  easiest  possible  one  and  takes  less  energy  than 
the  one  from  Pb3O4  or  PbO2  to  Pb.  No  hardening 
agent  is  needed,  for  the  negative  has  plenty  of  co- 
herence. But  it  does  need  porosity,  and  a  good 
many  makers  use  either  a  soluble  salt  like  magne- 
sium sulphate,  or  an  inert  substance  like  graphite, 
in  making  their  negative  paste.  It  seems  doubtful 
whether  the  effect  of  the  soluble  salt  is  lasting,  and 
there  seems  to  be  a  belief  that  graphite  and  the  other 
space-filling  inert  substances  which  are  suggested 
may  be  harmful  in  the  ordinary  open-grid  negative 
plate.  So  we  will  make  our  negatives  as  simply  as 
possible,  using  only  litharge  and  rather  dilute  sul- 
phuric acid,  and  allowing  the  plate  to  set  and  cement 


204 


STORAGE  BATTERIES 


very  much  as  though  it  were  to  become  a  positive. 
It  sulphates  to  the  amount  of  about  30  %  of  the 
whole  mass,  and  during  formation  the  changes  shown 
in  Figure  79  take  place.  In  this  case  the  plate  was 
about  20  %  sulphate  before  formation,  and  80  % 


40 


80  120  160  200 

AMPERE-HOURS  FORMATION 


£40 


Z80 


Fio.  79.  —  Changes  in  composition  in  a  paste  negative  plate  during 
formation. 

litharge.  Lead  begins  to  form  immediately  when 
the  current  is  started,  but  notice  how  the  sulphate 
content  also  rises  during  this  period  —  almost  as  fast 
as  lead  is  formed.  The  pores  are  opening.  Metallic 
lead  occupies  much  less  space  than  either  the  oxide 
or  the  sulphate,  and  the  acid  has  a  chance  to  reach 
and  attack  new  oxide  in  the  deeper  pores  of  the 
plate.  Before  long  the  sulphate  reaches  its  maxi- 


PASTE  PLATES  205 

mum,  and  then  it  seems  to  reduce  faster  than  it  is 
formed  from  the  oxide.  Finally  the  plate  stops 
when  it  contains  about  98  %  of  metallic  lead,  the 
rest  being  mainly  oxide,  with  a  very  small  remnant 
of  sulphate. 

Lead  sponge  made  in  this  way  is  tough,  coherent, 
and   well   interlocked    all    over    the    plate,    and   a 
properly  made  negative  has  a  chance  of  much  longer' 
life  than  the  positive  made  in  about 
the  same  way.     It  is  usually  said 
that  one  set  of  negatives  will  just 

about  OUtlast  two  Sets  Of  positives.       'IGpaste  negative. 

The   rites  of    negative    grids   are 
often  made  with  dovetails  as  shown  in  Figure  80,  the 
intention  being  to  hold  the  contracting  material  in 
better  contact  with  the  support. 

115.  "Chloride"  and  "Box"  Negatives.  —  Two  vari- 
ants on  the  usual  processes  have  been  of  importance. 
The  "  chloride  "  negative  was  made  by  casting  a  lead 
grid  around  pellets  made  from  molten  lead  chloride. 
The  whole  plate  was  then  reduced  to  sponge  lead, 
and  the  active  material  so  formed  had  many  good 
qualities.  This  process  is  no  longer  in  use.  The 
other  plate  in  this  class  is  the  "  box  "  negative,  origi- 
nated by  the  most  important  of  the  German  battery 
companies  and  now  used  in  this  country  by  the 
Electric  Storage  Battery  Company.  The  appear- 
ance of  the  finished  plate  is  shown  in  Figure  91. 


206  STORAGE  BATTERIES 

Pellets  containing  litharge  mixed  with  some  lamp- 
black or  other  "  expander  "  are  made  outside  of  the 
plate  and  dropped  into  place  in  the  openings.  They 
are  then  covered  by  the  other  sheet  of  perforated 
lead,  and  the  plate  is  complete.  This  particular 
active  material  has 'no  coherence  at  all,  and  would 
fall  out  of  the  openings  in  an  ordinary  grid  in  a  few 
days  of  service,  but  by  protecting  it  with  this  per- 
forated cover  it  can  be  made  to  give  good  capacity 
and  life. 


CHAPTER  XVI 

DISEASES  AND  TROUBLES 

116.  Frequent  mention  has  been  made  of  action 
between  the  peroxide  and  the  lead  support  in  the 
positive  plate,  resulting  in  self-discharge  proportional 
to  the  quantity  of  material  affected.  Lead  sulphate 
is  formed  at  the  surface  of  contact.  This  action  is  a 
perfectly  normal  part  of  the  activity  of  every  positive 
plate.  It  is  a  large  factor  for  the  original  flat  plates 
of  Plante,  fairly  large  —  quite  measurable  at  any 
rate  —  for  modern  large  surface  plates,  very  small  in 
paste  plates. 

While  this  action  is  a  normal  one,  and  essential 
in  its  nature,  it  may  be  so  exaggerated  by  wrong 
operating  conditions  that  it  becomes  a  source  of 
danger. 

Between  sponge  lead  and  solid  lead  the  difference 
of  potential  is  so  small  that  self-discharge  is  very 
slight.  But  in  many  of  the  modern  negative  plates 
there  are  other  things  than  lead.  Many  have  graphite 
in  them  to  give  contact,  insure  porosity,  and  make 
the  active  material  a  better  conductor.  With  this 
substance  in  the  negative  material  there  is  a  good 

207 


208  STORAGE  BATTERIES 

deal  of  local  action,  and  the  negatives  may  discharge 
themselves  quite  as  fast  as  the  positives  in  the  same 
cell. 

These  normal  effects  of  self-discharge  we  must 
take  with  our  storage  cell,  for  they  are  a  part  of  its 
nature.  There  are  many  other  substances  which 
might  be  in  the  cell  —  impurities  —  and  which  can 
greatly  increase  the  local  action.  Some  of  these  are 
so  strong  in  their  effects  that  they  are  dangerous  to 
the  life  of  the  cell. 

Suppose,  for  example,  that  a  very  stable  and  per- 
sistent forming  agent  has  been  used  in  the  manufac- 
ture of  the  plate  and  that  this  has  not  been  carefully 
removed  after  formation  and  before  the  plate  is  put 
into  service.  During  each  charging  period  this 
forming  agent  will  bore  into  the  peroxide  plate 
(anode)  and  continue  formation  at  a  rate  determined 
by  the  concentration  of  the  forming  ion.  From  our 
discussion  of  rapid  formation  it  will  be  remembered 
that  maximum  rapidity  of  formation,  and  density 
and  coherence  of  material  formed,  result  from  using 
a  definite  value  for  the  ratio  — 

concentration  of  forming  agent 
concentration  of  acid 

and  that  the  velocity  of  formation  dropped  very 
rapidly  when  the  concentration  of  forming  ion  was 
carried  much  below  the  value  indicated  by  this  ratio. 


DISEASES  AND   TROUBLES  209 

In  the  working  cell  there  is  not  much  likelihood  of 
enough  of  our  stable  and  persistent  forming  agent 
remaining  in  the  plate  to  approach  this  value.  If 
such  an  agent  were  present  at  anything  like  the 
optimum  concentration,  the  positive  plate  would 
have  a  total  life  of  only  a  few  cycles.  By  that  time 
the  lead  support  would  be  completely  peroxidized, 
and  the  plate  would  fall  to  pieces. 

Large  surface  plates  attain  a  life  of  1000  or  more 
discharges.  If  a  plate  is  to  compete  on  these  terms, 
even  a  minute  amount  of  forming  action  makes  a 
difference  in  results,  and  so  manufacturers  have 
learned  to  carefully  remove  the  forming  agent  before 
sending  their  plates  into  service. 

Another  thing  helps  very  much.  Most  active 
forming  agents  are  soon  completely  decomposed  by 
the  electrochemical  action  of  the  cell.  Nitric  acid 
has  been  frequently  used  as  an  active  forming  agent. 
It  is  reduced  to  ammonia  at  the  cathode  and  remains 
in  the  cell  only  as  a  slight  impurity  of  ammonium 
sulphate  in  the  electrolyte.  While  this  latter  sub- 
stance is  not  to  be  prescribed  as  good  for  the  cell  it  is 
not  actively  dangerous. 

This  danger  is  confined  to  the  peroxide  plate, 
and  the  most  unhealthy  impurities  are  the  forming 
agents  of  the  list  given  on  page  185.  Of  course  the 
dangerous  ions  turn  and  go  to  the  negative  (lead 
sponge)  plate  during  discharge,  but  the  voltage  is 


210  STORAGE  BATTERIES 

much  lower  and  the  plate  appears  well  able  to  pro- 
tect itself  by  a  layer  of  sulphate. 

117.  The  lead  sponge  plate  has  its  own  class  of 
uncomfortable  impurities  —  the  metals  —  and  they 
have  no  power  to  affect  the  life  of  the  plate.  They 
merely  cause  self-discharge.  This  they  do  by  set- 
tling on  the  plate  and  causing  little  local  cells. 
During  charge  the  lead  plate  is  catho/le.  All  the 
metallic  ions  in  the  cell  wander  over  to  this  plate, 
and  if  they  can  go  out  of  solution  at  the  voltage 
of  charge  and  under  the  existing  conditions  in  the 
cell,  they  deposit  as  metal  on  the  lead  plate.  Little 
cells 

metal/sulphuric  acid/lead 

discharge  as  soon  as  the  voltage  is  removed,  and 
the  current  used  in  their  discharge  is  lost  as  far  as 
external  work  is  concerned.  The  cell  appears  on 
test  to  have  lost  capacity. 

Evidently  the  noble  metals  will  be  the  chief  of- 
fenders, for  they  go  out  of  solution  very  readily  and 
give  a  local  cell  with  a  good  big  electromotive  force 
for  self -discharge.  A  very  little  platinum  will  keep 
a  negative  plate  from  taking  in  more  than  a  minute 
fraction  of  its  proper  charge.  This  unpleasant  effect 
does  not  persist  for  many  cycles  ;  for  while  the  noble 
metals  are  ready  enough  to  go  out  of  solution,  they 
are  not  ready  to  go  back  in  again.  At  any  rate, 


DISEASES  AND   TROUBLES  211 

when  the  lead  plate  is  cathode  (charge)  the  noble 
metal  goes  out  before  the  lead  does,  and  the  latter 
plates  it  over  and  eventually  covers  it  away  out  of 
reach.  As  the  negative  naturally  increases  the  size 
of  its  grain  in  service,  the  noble  metals  are  gradually 
incapsulated  in  the  heart  of  the  lead  grains,  which 
no  longer  react  completely  to  the  very  center  at  each 
reversal. 

Copper,  silver,  and  gold  can  act  in  the  same  way  as 
platinum.  Copper  is  not  very  active,  and  the  ac- 
tivity increases  to  the  other  end  of  the  list. 

118.  There  is  still  a  third  class  of  impurities  which 
can  cause  self-discharge,  though  its  representatives 
have  no  direct  effect  on  the  plates.  This  class  in- 
cludes those  ions  which  can  exist  in  two  stages  of 
oxidation  and  which  are  easily  converted  from  one 
state  to  the  other.  Iron  is  the  commonest  example. 
Suppose  a  workman  drops  a  pair  of  pliers  into  a 
storage  cell  during  its  installation.  When  the  elec- 
trolyte is  poured  into  the  cell,  these  pliers  dissolve 
gradually  to  form  ferrous  sulphate,  and  now  the  cell 
contains  Fe++  ferrous  ion.  This  travels  about  in  the 
cell,  and  during  discharge  it  migrates  along  with  the 
H+  to  the  cathode,  now  the  peroxide  plate.  When 
it  meets  with  lead  peroxide,  it  is  oxidized  to  Fe+++ 
ferric  ion.  Even  if  the  cell  is  on  open  circuit,  the 
action  will  take  place  as  fast  as  Fe++  reaches  the 
peroxide  plate,  and  as  soon  as  a  little  Fe++  has  been 


212  STORAGE  BATTERIES 

oxidized  to  Fe+++  a  slight  concentration  gradient  is 
set  up  which  hastens  the  motion  of  Fe++  toward  the 
peroxide  plate  and  the  removal  of  Fe+++  from  the 
neighborhood.  In  the  meantime  Fe+++  has  wan- 
dered over  to  the  lead  plate,  and  there  it  is  reduced 
to  Fe++,  setting  up  a  diffusion  gradient  there  in  the 
same  direction  as  the  one  at  the  other  plate.  Every- 
thing conspires  to  aid  in  the  discharge  so  produced. 
No  metallic  iron  is  deposited,  but  every  bit  of  Fe++ 
and  Fe+++  in  the  cell  keeps  busily  at  work  running 
from  one  plate  to  the  other  and  discharging  the  cell. 
Even  a  small  amount  of  pliers  in  a  large  cell  will 
cause  a  considerable  self-discharge  in  24  hr.  This 
is,  of  course,  an  effect  which  is  especially  noticeable 
on  open  circuit.  If  the  cell  is  working  hard,  charg- 
ing and  discharging  every  few  hours  or  every  few 
minutes,  the  loss  of  energy  will  be  negligible. 

Probable  Impurities.  —  The  list  includes :  — 

Forming  agent.     From  rapid  forming  process. 

Iron. 

Copper. 

Tin. 

Arsenic. 

Antimony. 

Platinum  (noble  metals  in  general). 

119.  A  certain  amount  of  depreciation  must  be 
expected  in  a  battery,  even  if  it  is  kept  in  the  best 
possible  condition.  The  effects  of  local  action  cannot 


DISEASES  AND   TROUBLES  213 

be  avoided,  nor  can  the  negative  active  material  re- 
tain its  original  porous  structure  throughout  the 
whole  life  of  the  cell.  Plates  shed  their  active  ma- 
terial. Positive  peroxide  loses  its  coherence  and  falls 
off  the  plate  even  in  the  case  of  the  toughest  of  Plante 
type,  and  to  a  much  greater  extent  in  paste  types. 

These  normal  disturbances  may  be  greatly  magni- 
fied by  poor  operating  conditions.  We  will  make  a 
list  of  the  common  diseases  which  are  especially  ap- 
parent in  Plante  types. 

1.  Loss  of  capacity.  —  This  is  due  to  wholly  differ- 
ent causes  in  positive  and  negative  plates.  A  Plante 
positive  should  retain  ijbs  capacity  almost  unchanged 
up  to  nearly  the  end  of  its  life.  It  has  great  power 
of  recuperation  and  can  re-form  lost  active  material 
and  should  remain  healthy  for  the  rate  at  which  it  is 
operating  if  it  is  carefully  handled.  Toward  the  end 
of  its  life  all  the  reserve  lead  will  become  exhausted. 
If  it  is  made  with  rosettes,  like  the  Manchester  type, 
all  the  pure  lead  in  the  strips  becomes  changed  into 
peroxide,  and  the  plate  then  becomes  like  a  rather 
low-surface  paste  plate.  The  grid  remains  unat- 
tacked,  but  the  capacity  has  reached  a  maximum, 
and  from  this  time  on  peroxide  will  be  shed  and  no 
more  can  be  formed  to  replace  it.  Events  follow 
much  the  same  course  in  a  ribbed  Plante  plate.  The 
ribs  will  become  entirely  peroxidized  and  the  main 
supporting  webs  have  not  sufficient  surface  to  keep 


214  STORAGE  BATTERIES 

up  the  supply.  The  ribs  finally  disappear,  as  do  the 
rosettes  of  the  Manchester  type.  The  plate  is  ap- 
proaching the  end  of  its  useful  life. 

120.  The  Plante  negative  has  a  more  peaceful  ex- 
istence and  an  almost  indefinite  life,  but  it  diminishes 
rather  rapidly  in  capacity  during  the  first  hundred 
cycles  or  so  of  service  and  continues  to  lose  more  and 
more  unless  it  is  regenerated  by  some  means.     This 
loss  of  capacity  has  been  spoken  of  before  (page  192).- 
It  is  due  to  the  increase  in  size  of   grain  and  the 
general  decrease  in  surface  which  results  from  many 
cycles  of  charge  and  discharge.      The  large  grains 
persist  and  are  not  completely  transformed  into  sul- 
phate during  discharge.     The  lead  deposits  on  them 
rather   than   to   form    new   grains.     Then,  too,    the 
smaller  grains  are  more  soluble  than  the  large  ones, 
and  these  two  effects  taken  together  combine  to  pro- 
duce a  continual  and  considerable  droop  in  capacity 
with  service.     One  way  to  bring  back  the  original 
condition  of  the  plate  is  to  completely  reverse  it  to 
peroxide  and  then  back  to  lead  again,  but  this  is  not 
very  frequently  feasible  in  practice,  where  the  plate ' 
is  set  up  with  many  other  positives  and  negatives  in 
a  large  cell. 

121.  Another  way  of  restoring  the  original  capacity 
of  a  Plante  negative  is  by  means  of  a  process  called 
"  Permanizing.  "     The  plate    is  soaked  in  a  rather 
strong  solution   of  sugar  and  then  heated  to  about 


DISEASES  AND   TROUBLES  215 

300°  C.  for  a  time.  The  sugar  is  quite  completely 
carbonized  at  a  point  below  the  melting  point  of  lead, 
and  the  pores  of  the  active  material  are  filled  with 
very  finely  divided  carbon.  This  carbon  prevents 
the  pores  from  filling  up  with  lead,  and  the  grains 
may  also  act  as  centers  on  which  lead  can  precipitate. 
At  any  rate,  plates  treated  in  this  way  seem  to  retain 
their  capacity  longer  than  usual,  and  a  plate  which 
has  lost  a  part  of  its  capacity  by  service  has  most  of 
it  restored  by  the  treatment. 

2.  Deformation.  —  All  Plante  plates  are  more  or 
less  subject  to  buckling  or  fracture.  If  they  are 
made  of  pure  lead,  they  twist  and  stretch  when  any 
strain  is  put  on  them,  and  if  they  are  made  of  anti- 
mony alloy,  they  are  liable  to  crack  instead.  In  the 
case  of  pure  lead  plates,  buckling  may  be  caused 
by  improper  formation.  If  one  side  of  the  plate  is 
formed  more  deeply  or  completely  than  the  other,  the 
changes  of  volume  which  occur  will  twist  or  bend  the 
soft  lead  and  the  plate  buckles.  Almost  all  Plante 
plates  with  ribs  grow  in  length  considerably  during 
formation,  and  if  the  resulting  peroxide  is  dense  and 
firmly  attached  to  the  lead  of  the  support,  the 
stretching  may  be  as  much  as  an  inch  or  more.  It 
is  almost  wholly  along  the  rib  —  much  less  marked 
across  the  plate ;  a  perfectly  normal  effect,  and  known 
and  allowed  for  by  all  manufacturers  who  make  this 
type  of  plate.  Lead  is  so  soft  a  metal  that  the 


216  STORAGE  BATTERIES 

material  produced,  which  is  greater  in  volume  than 
the  lead  from  which  it  is  made,  and  which  adheres 
strongly  to  the  surface,  exerts  force  sufficient  to 
stretch  the  whole  plate. 

Certain  operating  conditions  may  tend  to  cause 
buckling.  For  example,  if  a  battery  has  been  on 
very  high  rate  work,  its  ribs  and  pores  are  very  open. 
If  now  it  is  changed  over  and  put  on  low  rates,  espe- 
cially of  charge,  its  plates  are  very  liable  to  buckle. 
Much  new  peroxide  will  be  formed  away  down  near 
the  central  support  of  the  plate,  and  this  can  easily 
fill  the  available  space  between  ribs  too  full. 

And  sulphation,  in  the  evil  sense  of  the  word,  can 
cause  plates  to  tie  themselves  almost  into  knots. 
Here  the  change  of  volume  is  as  great  as  possible, 
and  all  the  pores  and  spaces  in  the  plate  are  over- 
crowded with  material.  It  may  be  taken  as  a  gen- 
eral rule  that  any  treatment  which  can  cause  more 
than  the  normal  change  of  volume  in  the  deeper 
active  material  of  the  plate  will  give  rise  to  buckling 
or  fracture. 

3.  Sulphation.  —  This  is  a  "  waste-basket  word  " 
among  all  the  people  who  have  to  deal  with  storage 
batteries.  Whenever  anything  whatever  seems 
wrong  with  a  cell,  the  first  diagnosis  is  usulphated." 
Lead  sulphate  usually  has  something  to  do  with  the 
difficulty,  but  its  connection  may  be  of  the  most  re- 
mote. The  most  common  cause  of  trouble  is  lack  of 


DISEASES  AND   TROUBLES  217 

proper  charge.  In  days  not  so  long  past,  batteries 
were  often  sent  out  a  long  way  into  the  country,  to 
a  point  miles  distant  from  the  power  house,  and 
allowed  to  "  float "  on  a  trolley  line  to  help  the  vol- 
tage and  save  copper  feeders.  These  lonely  batteries 
often  had  a  hard  time  as  far  as  proper  charge  was 
concerned,  and  some  of  them  furnished  examples  of 
sulphation  and  buckling  of  the  most  aggravated 
nature.  Engineering  practice  has  improved  since 
then,  and  boosters  and  feeders  have  been  found  eco- 
nomical compared  with  the  rapid  depreciation  of 
batteries  used  in  this  way.  In  the  case  of  station 
batteries  properly  operated,  there  is  not  nowadays 
much  cause  to  use  the  word  "sulphation." 

4.  Impurities  and  local  discharge.  —  Before  the 
danger  of  very  low  charging  rates  and  the  worse 
danger  arising  from  a  net  discharge  were  clearly 
appreciated,  many  of  the  troubles  with  plates  were 
sought  for  in  the  presence  of  "  impurities "  in  the 
cells.  Every  rapid  forming  agent  was  suspected, 
and  water,  acid,  and  even  air  were  examined  with 
great  care  for  possible  explanations  of  trouble.  It 
will  be  evident  from  what  has  been  said  about  the 
elimination  of  the  forming  agent  and  its  comparative 
action  in  very  dilute  solution  that  these  analyses  and 
examinations  were  without  positive  result.  A  stor- 
age cell  should  contain  nothing  but  sulphuric  acid; 
but  it  takes  a  long  time  to  accumulate  troublesome 


218  STORAGE  BATTERIES 

impurities  if  reasonably  pure  water  is  used  to  fill  the 
cells,  and  many  of  the  troubles  mentioned  appeared 
within  a  few  months  of  service.  It  seems  now  fairly 
certain  that  the  whole  effect  could  be  explained  by 
undercharge,  by  the  fact  that  the  plates  got  a  net 
discharge,  and  by  the  fact  that  the  charging  rates 
were  much  too  low.  Certainly  these  factors  can 
cause  sulphation  and  buckling,  and  even  destruction 
of  a  whole  battery,  in  the  way  these  troubles  used  to 
occur. 

5.  Shedding  of  active  material.  —  Plante  positives 
shed.  So  do  paste  plates,  but  the  shedding  is  a  more 
healthy  thing  for  the  Plante  plate,  and  is  a  part  of 
its  physiology.  On  page  174  there  was  pictured  the 
way  in  which  well-made  Plante  plates  adapt  them- 
selves to  the  rate  at  which  they  are  working.  No 
plate  can  do  this  so  well  as  the  simple  ribbed  type  of 
positive.  Even  the  Manchester  plate,  nearly  uni- 
versal in  its  application  though  it  may  be,  cannot 
compete  with  the  simple  ribbed  pure  lead  type  in 
adaptability,  and  especially  in  lively  response  to  the 
demands  of  very  rapid  rates.  At  low  and  inter- 
mediate rates  the  sensitive  pure  lead  plate  is  at  a 
disadvantage,  for  it  is  endangered  by  low  charge 
rates,  and  is  by  no  means  so  excellent  at  low  dis- 
charge rates  as  at  high  ones. 

Plante  negatives  have  none  of  these  weaknesses. 
Their  only  failing  is  the  one  already  described  — 


DISEASES  AND   TROUBLES  219 

rapid  loss  of  capacity.  As  far  as  health  and  tough- 
ness are  concerned,  they  are  beyond  criticism. 

6.  Short  circuits  in  the  cell.  —  The  almost  universal 
use  of  wood  separators  has  nearly  removed  this  once 
common  source  of  trouble.  Any  large  surface  plate 
develops  strips  and  flakes  of  surface  sulphate  or  other 
surface  material.  This  drops  off  and  sometimes 
reaches  across  from  positive  plate  to  neighboring 
negative.  Often  these  delicate  bridges  are  quite 
innocuous,  but  they  occasionally  become  formed  part 
way  or  all  the  way  across,  and  the  result  is  a  complete 
short  circuit  in  the  cell.  Local  action  may  be  very 
great  indeed  at  the  two  points  of  contact  of  this 
bridge,  and  many  a  plate  has  had  a  hole  eaten  right 
through  it  by  the  very  high  local  current  within  a 
few  days  after  the  accident  occurred.  Rigorous  in- 
spection is  the  only  way  to  avoid  such  an  accident, 
and  the  acid  density  is  the  very  best  indicator  of 
trouble.  In  small  glass  jars  it  is  easy  to  see  whether 
anything  has  occurred,  but  in  the  big  lead-lined  tanks 
used  for  large  batteries  it  would  be  a  great  deal  of 
work  to  look  down  between  each  of  the  ten  thousand 
or  more  pairs  of  plates  every  day.  If  the  cell  is  not 
working  properly,  its  acid  density  will  not  rise  during 
charge  to  the  proper  value,  and  this  may  always  be 
considered  a  sign  of  trouble. 

As  a  battery  grows  old  much  sediment  forms  in 
the  bottom  of  the  cells,  and  if  this  is  not  removed, 


220  STORAGE  BATTERIES 

the  plates  will  eventually  short-circuit  across  their 
bottom  edges.  Pure  carelessness  or  laziness  only  can 
account  for  such  a  condition. 

7.  General  debility.  —  The  "  storage  battery  man  " 
learns  to  judge  pretty  well  about  the  condition  of  a 
battery  by  looking  it  over.  "  She  don't  look  right," 
is  reason  for  a  careful  investigation.  If  a  battery 
has  been  doing  well  and  then  begins  to  show  signs  of 
ill  health,  an  examination  of  the  charge  and  discharge 
charts  will  usually  show  the  reason  for  the  change. 
Perhaps  the  station  has  been  called  on  for  heavier 
loads  during  a  period  of  two  weeks  or  so.  A  prime 
power  unit  may  have  been  out  of  commission  in  the 
station.  The  old  booster  may  not  be  large  enough 
for  the  work  to  be  done.  It  usually  turns  out  that 
the  battery  has  given  a  net  discharge,  or  else  the 
necessary  net  overcharge  cannot  be  given  in  the 
time  that  remains  after  the  hard  work  of  the  day. 
Some  such  cause  will  usually  be  found. 

122.  A  few  years  ago  I  had  hundreds  of  plates 
sent  to  me  for  chemical  analysis  from  batteries  where 
troubles  of  this  kind  appeared.  The  plates  and  the 
electrolyte  were  in  all  cases  as  pure  as  possible,  but 
in  most  cases  investigation  showed  that  the  battery 
was  being  charged  at  too  low  a  rate  and  not  fully 
charged  at  that ;  the  plates  had  buckled  and  turned 
in  color.  In  every  case  where  investigation  was  pos- 
sible operating  conditions  were  responsible,  but  it 


DISEASES  AND   TBOUBLES  221 

sometimes  took  careful  examination  and  even  diplo- 
macy to  bring  out  this  truth.  A  good  starting  point 
in  cases  like  this  is  the  maxim  "Look  at  the  rates 
under  which  the  battery  is  working."  If  a  compara- 
tively new  battery,  once  healthy  and  lively,  turns 
weak  and  sickly,  and  plates  begin  to  buckle  and  shed, 
do  not  suspect  "  impurities."  Suspect  operating  con- 
ditions. See  that  the  battery  is  charged.  See  that 
it  is  overcharged,  and  the  chances  are  large  that  all 
the  troubles  will  disappear. 

These  directions  are  sometimes  overdone,  but  not 
very  often  in  my  experience.  It  is,  of  course,  quite 
possible  to  overcharge  Plante  plates  until  almost  all 
the  active  material  is  blown  off  the  positive  plates 
by  continued  gassing.  But  few  superintendents  will 
allow  their  battery  men  to  waste  current  in  this  way. 
Oftener  they  are  obliged  to  beg  for  enough  charging 
current  to  keep  the  battery  in  good  condition. 

A  well-made  large  surface  plate  seems  to  love 
work.  No  battery  looks  so  healthy  (to  me  at  least) 
as  one  which  has  stripped  itself  for  service,  at,  say, 
the  20-min.  rate  or  better.  The  plates  look  lean, 
but  their  color  is  good.  They  do  not  gas  very  much 
except  at  the  very  end  of  charge.  The  current  which 
can  be  drawn  from  such  a  battery,  especially  when 
it  is  installed  in  a  warm  place,  is  astonishing.  In 
earlier  days  the  8-hr,  rate  was  "normal."  In  pres- 
ent-day service  the  5-min.  rate  is  more  nearly  the 


222  STORAGE  BATTERIES 

rate  at  which  the  battery  is  most  useful.  There  is  a 
good  reason  for  this.  Suppose  our  battery  can  give 
100  amperes  for  8  hr.  So  can  a  10  KW.  110-volt 
generator.  This  battery  can  give  3000  amperes  for 
a  minute  or  so.  It  would  take  fifteen  or  twenty 
generators  to  safely  handle  such  a  peak. 

123.  After  the  catalogue  of  ills  just  recited  it 
might  seem  that  the  lead  battery  must  be  given  up 
as  a  bad  job.  But  we  have  been  acting  in  the  role 
of  the  pathologist  in  this  case,  and  as  a  matter  of 
fact  the  lead  cell  is  a  pretty  healthy  and  lively 
machine,  if  it  is  well  treated.  Even  under  rather 
adverse  conditions  it  often  shows  surprising  powers 
of  resistance.  In  our  own  laboratory  we  have  cells  in 
use  which  are  over  twelve  years  old.  This  battery  has 
had  occasional  periods  of  a  few  months  each  of  hard 
service,  with  long  rests  between.  The  rests  have 
probably  been  harder  on  the  plates  than  the  work, 
for  it  has  sometimes  been  left  pretty  well  discharged, 
and  the  results  have  shown  themselves  in  disintegra- 
tion of  the  negative  plates. 

In  easy  service  the  life  of  positive  plates  should  cer- 
tainly Teach  six  years,  and  that  of  negatives  is  much 
longer.  In  stand-by  service  positives  may  last  ten 
years  and  negatives  twelve  or  fifteen.  In  hard  regu- 
lation work  the  positive  life  is  three  to  five  years  and 
negative  life  five  to  eight. 

Paste  plates  in  service  are  much   shorter  lived. 


DISEASES  AND   TROUBLES  223 

Probably  about  300  to  350  cycles  for  the  positives 
and  about  400  to  500  for  the  negatives  may  be 
taken  as  the  average  life.  In  stand-by  service  there 
seems  to  be  no  reason  why  the  life  should  not  be 
nearly  as  long  as  for  Plante  plates.  Local  action  is 
much  less  effective,  and  the  battery  is  kept  well 
charged. 

124.  It  is  possible  to  give  some  general  rules  for 
the  operation  of  batteries.  For  Plante  plates  :  — 

1.  Keep  the  battery  charged.         « 

2.  Charge   at   a   fairly  high   rate.      Usually  this 
means  at  the  8-hr,  rate  or  a  little  higher. 

3.  Inspect    frequently   and    remove   all   possible 
short  circuits  immediately. 

4.  Keep  acid  density  at  the  proper  point. 

5.  Keep  the  acid  above  the  top  of  the  plates. 

6.  If  plates  buckle,  straighten  them   as   soon  as 
possible. 

7.  Do  not  let  the  temperature  reach  too  high  a 
point.     (100°  F.  is  a  safe  limit.) 

Discharge  at  almost  any  rate  does  not  harm  good 
Plante  plates  provided  they  are  charged  immediately 
after  the  discharge  is  finished. 

For  paste  plates  :  — 

1.  Charge  at  a  low  rate,  12  hr.  or  lower. 

2.  Overcharge  occasionally  by  10  %  or  so.     Once 
a  week  is  often  enough  for  the  overcharge   if   the 
battery  is  in  daily  service. 


224  STORAGE  BATTERIES 

3.  Use  an  ampere-hour  meter  and  regulate  charge 
and  discharge  by  that. 

4.  Try  to  give  a  nearly  complete  discharge  be- 
fore recharging.     If  the  discharge  is  extended  over 
two  or  three  days,  no  harm  is  done. 

5.  Watch  temperature  carefully.     High  tempera- 
ture is  much  more  destructive  to  paste  plates  than 
to  Plante  types. 

6.  Test   each   cell  frequently  and  inspect  at  the 
least  sign  of  trouble. 

The  most  usual  trouble  arises  from  continued  net 
undercharge,  especially  in  private  installations. 


CHAPTER   XVII 
SOME  COMMERCIAL  TYPES 

125.  The  most  important  services  performed  by 
storage  batteries  are  in  regulation  of  large  station 
loads  and  as  "  stand-by  "  batteries.  The  work  per- 
formed in  these  two  applications  is  wholly  different, 
and  there  is  a  very  evident  movement  toward  the  use 
of  quite  different  types  of  plates  in  the  two  kinds  of 
service. 

Regulation  (Trolley  Service,  Large  Factory  Service, 
etc.).  —  The  battery  is  used  in  conjunction  with  a 
large  power  plant  and  often  with  a  "  booster."  The 
charge  and  discharge  rate  vary  from  five  minutes  to 
twenty  seconds  or  so.  This  is  the  hardest  and  most 
wearing  service  that  a  battery  can 'be  called  on  to 
perform,  and  it  is  the  most  important  from  the  point 
of  view  of  economy.  High  service  Plante  plates  are 
eminently  fitted  for  the  work,  and  paste  plates  are 
quite  out  of  their  element. 

A  very  large  number  of  patents  have  been  taken 

out  on  plates  of  the  Plante  type,  and  most  of  them 

have  dealt  with  the  methods  of  increasing  the  surface 

of  the  plate  or  with  the  method  of  forming  it.     Not 

Q  225 


226 


STOBAGE  BATTERIES 


many  of  the  really  marked  variations  have  met  with 
commercial  success,  and  gradually  practice  has  left 


FIG.  81.  —  Cross  section  and  sectional  view  of  a  "  Tudor  "  plate. 

only  a   very   few   really  fundamental   Plante   plate 
types. 

The  fundamental  intention  is  to  increase  the  active 
surface  of  the  plate  by  forming  ribs.  This  develop- 
ment of  the  surface  is  carried  out  before  formation 

with  a  rapid  forming  agent. 
The  Tudor  plate  may  be 
taken  as  type  (Figures  80 
and  81).  It  is  made  by  cast- 
ing pure  lead  in  a  mold  of 
proper  shape,  and  is  prob- 
ably the  best  known  and 
most  generally  used  of  all 
European  plates. 

Other  means  than  casting 


FIG.  82.  —  "  Tudor 
plate. 


positive 


are    also    used   to    produce 


SOME  COMMERCIAL   TYPES 


227 


the   same   increase   of   surface.      The   Gould   plate 
(Figures  83  and  84)  is  made  from  pure  sheet  lead 


FIG.  83.  —  Section  and  cross  section  of  "  Gould  "  plate. 

by  a  process  of  "  spinning."    The  sheet  of  lead  is  fed 
back  and  forth  between  rapidly  rotating  mandrels 


Jiniiiuunn 


. 

Ill/ 


FIG.  84.  —  Steps  in  the  spinning  of  a  "  Gould  "  plate. 

filled  with  steel  disks  spaced  far  enough  apart  to  give 
the  right  strength  of  rib  for  any  particular  service. 


228 


STORAGE  BATTERIES 


FIG.  85.  —  Cross  section 
of  "Gould"  plate 
[drawn  to  scale]. 


The  National  plate  (Figure  86)  looks  much  like  the 
Tudor,  but  is  made  by  swaging  ribs  and  webs  from  a 
sheet  of  pure  lead  instead  of  by 
casting.  Other  plates  very  simi- 
lar in  final  appear- 
ance are  made  by 
plowing,  by  pressing 
sheet  lead  through  a 
die  under  great  pres- 
sure, and  in  various 
other  ways. 

One  of  the  varia- 
tions, and  one  of  the 
oldest  and  most  gen- 
erally used,  is  the  "Manchester"  posi- 
tive, shown   in    Figure  87   and  already 
frequently  mentioned  in  the  more  the- 
oretical part  of  this  book.     This  is  not 
a  very   high   surface   plate,   but   it  has 
shown  itself  well  fitted  for  almost  every 
kind  of  work.     As 
will   be    seen   from 
the  cut,  the  active 
material  is   formed 
from  "  rosettes  "  of 
lead   ribbon,  and 


FIG.  86.  —  Longitudinal  and   cross  sec- 
tions of  a  "  National "  plate. 


these  are  pressed  into  a  cast  frame  of  antimony  lead 
before  formation.     The  frame  is  so  stiff  that  buck- 


SOME  COMMERCIAL   TYPES  229 

ling  should  not  take  place  except  under  extreme  ill 
treatment,  and  the  surface  is  sufficient 
for  any  except  the  very  highest  rates.  It 
is  perhaps  not  quite  so  efficient  at  high 
rates  as  the  plates  with  larger  developed 
surface  (Tudor, 
Gould,  National), 
but  the  latter  de- 
mand rather  more 
care  in  operation. 

B 

"t  The  Gould  plate 
\   (Figure  88)  has 

03* 

sg   the   longest   ribs 
"°    of    any    of    the 

.2  types  and  its  SUr-  FIG.  87.  — "Manchester" 
fl  /.  -,  positive. 

§   face  is  very  large 

"°  in  proportion  to  its  area.  This  is  with- 
•|  out  question  the  plate  most  responsive 
-%  in  high -rate  work,  and  most  efficient  in 
•J  the  hardest  service,  but  the  greater  sur- 
OD  face  and  longer  rib  mean  greater  inher- 
J.  ent  danger  from  local  action  and  greater 
*  probability  of  buckling  unless  operating 
£  conditions  are  closely  watched. 

It  is  perfectly  feasible  to  operate  any 
of  these  high-surface  batteries  at  aston- 
ishing rates,  and  in  modern  installations 
it  is  usually  the  booster  which  limits  the 


230  STORAGE  BATTERIES 

battery  discharge.  Most  manufacturers  are  quite 
willing  to  send  their  batteries  out  to  work  at  the 
5-min.  or  even  the  1-min.  rate  of  discharge.  A 
glance  at  the  table  will  show  what  sort  of  an  "  over- 
load" this  is,  if  the  term  has  any  application  to  a 
storage  battery. 

"  Normal  rate  "        1  for  8  hr. 

x    2  for  3  hr. 

x    4  for  1  hr. 

X    8  for  20  min. 

X  16  for  5  min. 

X  32  for  1  min. 

Of  course  the  term  "  normal "  as  applied  to  the 
8-hr,  rate  has  lost  significance,  since  the  most  im- 
portant work  of  the  battery  is  nowadays  performed 
at  a  very  much  higher  rate,  and  batteries  of  large 
size  are  not  often  put  in  for  service  at  this  rate  ex- 
cept for  stand-by  or  insurance  purposes.  The  20- 
min.  rate  is  more  nearly  "  normal "  in  modern  battery 
practice. 

In  regulation  work,  batteries  are  usually  operated 
in  conjunction  with  a  large  power  plant.  The  cells 
have  each  seventy-five  to  a  hundred  plates  about 
15  x  31  in.,  or  18  x  18  in.  (See  Figure  89.)  Each 
15  x  31  in.  positive  plate  gives  40  amperes  for  8  hr., 
and  from  this  the  capacity  of  the  battery  at  various 
rates  can  easily  be  calculated.  Suppose  each  cell  has 
101  plates. 


SOME  COMMERCIAL   TYPES  231 

50  positives  x  40  =     2000  amperes  for  8  hr. 

or     4000  amperes  for  3  hr. 

or     8000  amperes  for  1  hr. 

or  16,000  amperes  for  20  min. 

or  32,000  amperes  for  5  min. 

If  the  battery  is  working  in  conjunction  with  a 
500-volt  power  circuit,  it  will  consist  of  about  260 


E.SB.Co.449. 


FIG.  89.  —  One  cell  of  a  large  regulating  battery. 

cells.     The   power   obtainable  from   the  battery  is 
therefore 

2000  amperes  at  500  volts  = 

1000  KW.  for  8  hr. 
and  from  this  on  up  to 
32,000  amperes  at  about  400  volts  = 

12,000  KW.  for  5  min. 


232  STORAGE  BATTERIES 

Such  a  battery  would  only  be  used  in  connection 
with  a  very  large  power  plant  —  say  of  5000  KW. 
or  more. 

It  will  be  quite  evident  how  such  a  battery  should 
be  used.  Its  little  1000  KW.  would  hardly  be  felt 
at  the  8-hr,  rate,  but  its  12,000  KW.  can  give  reg- 
ulation of  enormous  short  peaks.  For  momentary 
peaks,  lasting  only  a  fraction  of  a  minute  at  their 
maximum,  this  battery  could  furnish  up  to  25,000 
KW. 

As  a  matter  of  fact  the  total  quantity  of  energy 
furnished  by  a  single  discharge  of  this  battery  is  not 
very  large,  as  measured  by  modern  requirements. 
It  can  give 

1000  x  8    =  8000  KW.H. 
if  discharged  at  the  8-hr,  rate,  and 

12,000x^2  =1000  KW.H. 

if  discharged  at  the  5-min.  rate. 

Its  main  importance  lies  in  its  power  to  absorb 
and  give  up  very  large  quantities  of  energy  in  very 
short  times  without  danger  to  itself  or  trouble  to  any 
one  about  the  station. 

"Stand-by"  or  Insurance  Batteries.  — The  most  im- 
portant of  all  the  applications  of  the  storage  battery 
is,  strange  to  say,  the  one  in  which  it  is  called  upon  to 
do  the  least  actual  work.  This  is  as  a  mere  reserve  of 
power,  to  be  used  only  in  case  of  emergency. 


234  STORAGE  BATTERIES 

It  is  of  the  utmost  importance  that  the  supply  of 
light  and  power,  in  a  city  or  in  any  large  service, 
should  be  continuous.  The  central  power  stations 
of  a  city  supply  thousands  of  consumers  in  every 
possible  application  of  electric  power.  Lights,  heat, 
machinery  of  every  description,  elevators,  —  all  de- 
pend on  the  continuous  service  given  by  the  power 
company.  Any  accident  which  resulted  in  stopping 
the  supply  of  energy,  even  for  a  few  minutes,  would 
do  a  lot  of  damage  and  inconvenience  many  people. 
The  stopping  of  all  the  generators  in  one  of  the  New 
York  stations  would  leave  thousands  of  people  in  the 
dark,  without  elevator  service,  with  no  work  to  do 
because  all  the  machinery  in  the  factory  was  dead. 

The  great  supply  companies,  like  the  various 
Edison  Companies  of  the  country,  take  every  pre- 
caution to  prevent  such  a  stoppage  in  service.  En- 
gines, turbines,  generators,  —  all  are  installed  in 
separate  units,  each  of  which  has  only  a  fraction  of 
the  work  of  the  station.  Enough  extra  sets  are  pro- 
vided to  allow  for  all  necessary  repairs  and  replace- 
ments *  without  interruption  of  service.  At  the 
bottom  of  all  these  precautions,  the  power  house  has 
connected  with  it  a  huge  storage  battery,  which  is 
kept  constantly  charged  and  which  is  called  on  for 
active  service  only  in  case  of  the  utmost  need. 

The  engineer  in  charge  of  the  station  has  taken 
every  precaution  and  has  provided  for  every  possible 


SOME  COMMERCIAL   TYPES  235 

emergency.  But  if  anything  should  happen  which 
puts  the  power  house  out  of  action  for  a  time,  the 
battery  is  big  enough  to  carry  the  whole  station 
load  for  a  few  minutes  —  long  enough  to  get  aid 
from  neighboring  stations  or  to  make  rapid  repairs 
and  changes.  The  battery  is  the  only  source  of 
power  which  is  wholly  reliable.  There  are  no  mov- 
ing parts,  and  there  are  no  high  pressures  to  cause 
trouble. 

One  of  these  stand-by  batteries  may  cost  1200,000 
and  be  called  on  for  only  two  or  three  real  discharges 
a  year.  Interest  and  depreciation  is  perhaps  $25,000 
a  year,  and  so  these  discharges  cost  $12,000  apiece, — 
a  couple  of  dollars  per  kilowatt-hour;  but  quite 
worth  the  price,  for  the  station  was  able  to  continue 
uninterrupted  service.  The  battery  pays  for  itself 
in  "  good  will "  alone. 

For  this  particular  class  of  service  the  manufac- 
turers are  beginning  to  use  a  new  class  of  plate.  As 
far  as  life  and  capacity  under  high  rate  is  concerned, 
the  large  surface  Plante  plate  is  of  course  the  best, 
and  many  stand-by  batteries  of  this  type  are  in  use. 
But  they  are  expensive  to  make.  Local  action  is 
considerable,  and  this  may  be  especially  true  at  the 
very  low  charging  rate  at  which  it  is  often  necessary 
to  charge  such  "a  battery.  Paste  plates  can  do  this 
work  quite  as  well  as  the  more  expensive  Plante  bat- 
tery. They  hold  a  charge  longer,  and  work  best  on  low 


236  STORAGE  BATTERIES 

charge  rates.  The  life  of  a  paste  plate  battery  is 
quite  sufficient  and  its  efficiency  is  good. 

Figure  90  shows  a  large  stand-by  battery  of  paste 
plates.  The  experience  of  European  manufacturers 
has  shown  that  such  batteries  are  economical,  and  we 
have  finally  come  round  to  using  paste  plates  for 
this  work,  but  about  ten  years  behind  the  practice  in 
Europe. 

126.  Negative  Plates.  —  Only  a  few  manufacturers 
use  the  true  Plante*  type  of  negative  plate  for  any 
service  whatever.  The  Gould  plates  are  the  only 
pure  Plante  negatives  in  general  use  in  this  country. 
The  negative  differs  from  the  positive  in  having  thin- 
ner ribs,  and  a  thinner  center  web,  and  in  having  a 
much  larger  percentage  of  the  whole  weight  in  the 
form  of  active  material.  It  is  made  by  formation  as 
positive  first,  and  the  rapid  forming  process  is  carried 
on  until  the  lead  of  the  original  blank  is  nearly  all 
changed  to  peroxide,  just  enough  being  left  to  hold 
the  plate  together.  There  is  no  danger  of  the  plate 
ever  getting  any  weaker  after  it  goes  into  service,  for 
once  it  has  been  reversed  to  the  negative  condition 
there  will  never  be  any  further  action  on  the  lead  of 
the  support  plate. 

Paste  Negatives.  —  The  commonest  type  of  negative 
plate  for  general  service  is  a  paste  plate.  It  differs 
from  the  negatives  used  in  electric  automobiles  only 
in  being  more  heavily  constructed.  The  grids  for 


SOME  COMMERCIAL   TYPES 


237 


these  plates  are  usually  made  with  the  dovetails  of 
the  strips  expanding  outward  to  give  the  active  ma- 
terial, which  contracts  in  service,  an  opportunity  to 
keep  in  good  contact  with  the  grid.  I  am  not  at  all 
sure  that  this  is  anything  more  than  an  inherited 
idea,  but  it  seems  to  be  followed  universally  by 
manufacturers  of  paste  negatives. 

Box  Negative.  —  The  Plante  negative  is  peculiar  in 
its  ways,  and  not  always  easy  to  control.  The  paste 
negative  has  not  the  ex- 
tremely tough  constitution 
necessary  for  some  of  the 
modern  high-rate  regulation 
work.  As  a  mean  between 
the  two,  and  with  the  inten- 
tion of  avoiding,  if  possible, 
the  troubles  of  both  the  other 
types,  what  is  called  the 
"box"  negative  has  been  de- 
veloped, and  put  into  active 
service  both  in  Europe  and  in  this  country.  It  is 
shown  in  Figure  91,  and  it  consists  of  a  frame  of 
antimony  lead  into  which  are  pat  the  blocks  of  active 
material.  A  front  and  back  cover,  both  full  of  fine 
perforations,  complete  the  plate.  The  active  ma- 
terial is  prepared  in  the  form  of  blocks  which  fit  the 
openings  in  the  frame.  Some  manufacturers  have 
sent  them  out  into  service  without  any  preliminary 


FIG.  91. —  "Box"   negative 
plate. 


238  STORAGE  BATTEEIES 

formation,  the  charge  necessary  for  the  development 
of  the  positives  being  just  about  sufficient  to  form 
the  very  porous  active  material  of  the  negative.  It 
is  usually  considered  better  to  form  them  before 
sending  them  out. 

At  first  sight  it  seems  like  a  decided  step  backward 
to  place  active  material  inside  a  box,  forcing  diffusion 
to  take  place  through  small  openings.  But  the  much 
more  difficult  diffusion  through  the  fine  pores  of  the 
material  inside  the  perforated  cover  completely  over- 
shadows any  effect  of  the  outside  cover.  Further- 
more, the  presence  of  the  cover  permits  the  maker  to 
use  a  very  porous  active  material  indeed.  It  need 
have  no  coherence  in  the  mechanical  sense  as  long  as 
it  has  conductivity,  and  the  latter  property  is  aided  by 
adding  finely  divided  carbon  to  the  prepared  block. 
The  increased  porosity  which  can  be  attained  in  this 
way  more  than  makes  up  for  the  longer  diffusion 
path  through  the  perforations  in  the  plate. 

127.  Submarine  Cells.  —  Next  in  order  of  size  after 
the  central  station  and  regulating  batteries  come 
the  ones  used  in  submarine  boats.  Here  the  design 
is  most  exacting,  for  both  space  and  weight  are 
sharply  limited,  especially  the  former,  and  a  very 
large  amount  of  power  must  be  furnished  over  a  con- 
siderable time.  Paste  plates  are  the  rule,  and  the 
average  size  is  about  15  x  24  in.,  and  from  21  to  35 
plates  to  the  cell.  The  containing  tanks  are  of  hard 


SOME  COMMERCIAL   TYPES  239 

rubber,  —  much  like  giant  vehicle  cells,  —  and  they 
are  fitted  with  arrangement  for  disposal  of  all  gases 
formed  during  operation.  The  mixture  of  hydrogen 
and  oxygen  which  is  produced  in  the  cell  is  about  as 
sharply  explosive  as  anything  possibly  could  be,  and 
serious  accidents  have  resulted  from  faulty  gas  dis- 
posal and  ventilation.  The  best  way  seems  to  be  to 
fit  each  cell  with  its  own  tight  cover  and  with  escape 
pipe,  rather  than  to  shut  up  the  cells  in  a  gas-tight 
compartment,  which  is  freed  from  gas  by  a  fan. 

The  plates  for  this  service  are  made  to  have  a 
capacity  as  high  as  is  compatible  with  a  reasonable 
life.  Tests  include  not  only  capacity  at  various  rates 
of  discharge,  but  also  tests  for  mechanical  strength, 
and  a  discharge  while  the  cell  is  being  rocked  rather 
violently  through  an  angle  of  about  30°. 

Of  course  the  boat  is  dependent  wholly  on  its 
batteries  for  power  while  submerged.  Sixty  cells 
must  give  about  5000  ampere-hours  at  the  3-  or 
4-hr.  rate.  Even  this  only  means 

110  x  5000 
3x746 

which  is  not  a  very  large  amount  of  power  to  drive 
a  boat  as  large  as  a  modern  submarine. 

128.  Train-lighting  and  Car-lighting  Service.  —  In 
Denmark  cars  have  been  carrying  batteries  for  light- 
ing service  for  more  than  twenty  years,  and  they  have 


240  STORAGE  BATTERIES 

found  this  application  a  valuable  one.  This  branch 
of  storage  battery  engineering  has  been  of  increas- 
ing importance  in  this  country  in  the  past  few  years. 
Some  day  before  long  it  will  be  statutory  that  every 
railroad  train  shall  do  all  its  lighting  by  electricity. 

The  simplest  system  is  "straight  battery."  The 
charged  battery  is  taken  on  at  one  terminal,  dis- 
charged at  a  rather  low  rate  during  the  trip,  —  at  the 
24-48  hr.  rate  —  and  removed  at  another  point,  a 
freshly  charged  battery  taking  its  place.  There  is 
much  of  this  practice  in  the  United  States.  The 
regular  cell  for  this  work  can  give  about  250  to  350 
ampere-hours.  Sixty  cells  in  a  battery  give  an  aver- 
age of  110  volts,  and  will  run  60  16-candle-power 
lamps  for  24  hr. 

Car-lighting  Systems.  —  Often  an  axle-driven  dynamo 
is  added,  which  can  furnish  somewhat  more  than 
power  enough  to  run  all  the  lamps  when  the  train  is 
moving  at  a  speed  greater  than  thirty  miles  per  hour. 
The  excess  energy  is  absorbed  by  the  battery  when 
the  train  is  running  at  higher  speeds  than  this,  and 
the  battery  must  run  the  lights  while  the  train  is 
standing  still.  Usually  a  complete  system  of  regula- 
tion is  provided,  so  that  the  battery  acts  just  as  a 
large  regulating  battery  would  in  a  power  plant  — 
absorbing  energy  whenever  an  excess  is  being  turned 
out  by  the  dynamo  and  giving  it  out  again  at  the 
times  when  the  speed  is  low  or  the  car  is  standing  still. 


SOME  COMMERCIAL    TYPES  241 

Train-lighting  Systems.  —  In  through  trains  which 
make  a  run  of  many  hours  without  change  in  make-up, 
the  generator  for  the  whole  train  is  sometimes  in- 
stalled on  the  locomotive  and  driven  by  a  steam  tur- 
bine. A  regular  "booster"  outfit  is  installed  either 
on  the  tender  or  in  the  baggage  car,  and  this  attends 
to  regulation  of  all  load  variations.  The  battery  in- 
stalled in  each  car  is  sufficient  in  capacity  to  run  its 
own  lights  for  a  time,  and  the  train  can  therefore  be 
made  up  and  broken  up  without  interruption  in  ser- 
vice. As  soon  as  the  train  has  been  made  up,  the 
generator  takes  the  load  and  the  batteries  are  kept 
nearly  fully  charged.  They  then  have  to  care  only 
for  the  regulation  and  to  serve  as  reserve. 

In  all  of  these  different  kinds  of  lighting  service, 
the  pure  Plante  plates  have  done  well,  and  most  of 
the  companies  who  do  this  work  make  special  Plante 
type  plates  for  it. 

129.  Vehicle  Service.  —  A  rapidly  growing  field  of 
usefulness  for  the  storage  battery  is  in  vehicle 
service.  At  first  glance  it  seems  a  poor  substitute 
for  the  light  and  efficient  internal  combustion 
engines  of  modern  times.  To  drive  a  pleasure 
vehicle  at  a  reasonable  speed  over  average  streets 
and  good  roads  requires  about  1.5  KW.  If  the 
battery  has  32  cells,  its  average  voltage  during  dis- 
charge will  be  60,  and  each  cell  must  be  able  to 
give  25  amperes  for  four  or  five  hours.  Such  a  bat- 


242 


STORAGE  BATTERIES 


tery  will  cost  about  $250,  and  will  weigh  not  far 
from  750  Ib.  complete. 

But  this  electric  vehicle  has  many  important  ad- 
vantages. It  is  clean  and  neat,  it  is  simple  to  oper- 
ate, and  it  is  almost  absolutely  certain  to  go  if  there 

is  a  charge  in  the  battery. 
Where  a  central  charging 
station  can  arrange  to 
charge  many  batteries  each 
night,  the  whole  arrange- 
ment is  efficient  and  eco- 
nomical. It  is  rather  strange 
to  see  how  the  heavy  truck, 
driven  by  electricity  and 
doing  its  hard  work  day 
after  day,  has  been  the  best 
of  arguments  with  which  to 
convince  the  doubter  of  the 
economy  of  the  electric  ve- 
hicle in  light  work  and  for 
pleasure. 

The  plates  used  in  vehicle  work  are  legion  in  name 
and  varied  as  to  fame.  Paste  plates  are  now  almost 
universally  used  over  the  world.  European  practice 
runs  toward  thinner  and  lighter  plates,  cheaply  made 
and  with  a  limited  but  well  understood  life.  In  this 
country  we  make  heavier  and  stronger  plates  of 
lower  weight  capacity,  but  having  longer  life. 


FIG.  92.  —  Paste  vehicle  grid. 


SOME  COMMERCIAL   TYPES  243 

WEIGHT  EFFICIENCY  OF  PASTE  BATTERIES 

American  Standard  Plates  7^-8|  watt-hours  per  pound 

American  Light,  high  capacity,  10£  watt-hours  per  pound 

Edison  12|  watt-hours  per  pound 

Medium  European  11  watt-hours  per  pound 

Light  European  14  watt-hours  per  pound 

Figures  93  and  94  show  one  of  the  commonest 
types  of  grids  used  in  making  vehicle  plates.  Most 
positive  grids  are  so  made  as  to  give  support  from 
outside  to  the  rather  loose  and  noncoherent  per- 
oxide. This  support  is  supposed  to  be 
given  by  making  the  ribs  of  the  cross- 
section  shown  in  Figure  76.  The  neg- 
ative grid  is  made  with  its  dovetail  in 
the  opposite  sense,  as  already  explained. 
Many  complicated  forms  of  grid  have 
been  patented  and  used,  but  gradually 
the  majority  of  manufacturers  have 
settled  down  to  the  similar  types. 

The  old  original  ideas  are  sure  to  recur  FIG.  93.  —  Cross- 
section  of  mold 
at  fairly  regular  intervals,  sometimes      and  grid  cast- 

because  the  cause  of  trouble  has  been 
removed,  and  sometimes  because  it  has  been  forgotten. 
About  the  only  decided  variation  from  the  simple 
grid  type  now  in  evidence  is  the  so-called  "  iron- 
clad"  vehicle  plate  (Figure  95).  The  type  is  pe- 
culiar in  depending  on  an  insulating  support  grid  or 
envelope  of  rubber,  celluloid,  porous  biscuit  ware, 


244 


STORAGE  BATTERIES 


wood,  etc.  This  surrounds  the  active  material  and 
prevents  shedding,  and  contact  is  made  with  a  cen- 
tral lead  strip  or  wire.  There  seems  every  reason 

to  believe  that  the  apparent 
security  is  not  a  very  real 
one.  It  is  quite  possible 
for  positive  active  material 
to  lose  coherence  and  ca- 
pacity even  though  the  ma- 
terial cannot  get  away  and 
fall  to  the  bottom  of  the 
cell,  as  it  does  in  the  or- 
dinary case. 

This  particular  plate 
has,  however,  been  care- 
fully tested  by  the  makers, 
and  may  prove  an  excep- 
tion to  the  rule. 

The  present  status  of 
the  vehicle  battery  might 
be  summarized  as  follows :  There  is  not  very  much 
difference  in  standard  plates  by  different  makers. 
Grids  differ  but  slightly.  Formation  and  other  treat- 
ment is  becoming  a  well-known  art.  With  proper 
operation  the  good  American  battery  should  give 
250  to  400  cycles  without  much  trouble.  It  must 
be  cleaned  once  during  this  life,  probably  after  200 
to  300  cycles. 


FIG.  94.  —  Type    of    grid  for 
paste  positive. 


SOME  COMMERCIAL   TYPES 


245 


If  operating  con- 
ditions are  not  right, 
the  same  battery 
may  begin  to  give 
trouble  after  100 
discharges  or  less. 
I  know  of  one  com- 
pany which  man- 
ages to  get  nearly 
450  cycles  in  hard 
service  from  any  one 
of  several  of  the 
standard  American 
types. 

A  set  of  vehicle 
negative  plates  is 
usually  assumed  to 
outlast  two  sets  of 
positives.  This  is 
usually  conserva- 
tive. 


FIG.  95.  —  "  Iron-clad"  vehicle  plate. 


CHAPTER   XVIII 
ACCUMULATORS  IN   GENERAL 

130.  It  is  both  strange  and  interesting  that  so 
few  galvanic  combinations  have  been  found  which 
are  really  fit  for  use  as  accumulators.  Plante  began 
his  experiments  in  1859,  at  a  time  when  the  whole 
scientific  world  was  much  interested  in  the  subject 
of  galvanic  cells,  and  he  worked  for  more  than  thirty 
years  on  the  problem.  During  that  time  an  immense 
number  of  combinations  were  suggested  for  use  as 
primary  cells,  but  hardly  a  new  idea  was  brought 
forward  for  the  improvement  of  secondary  cell,  with 
the  exception  of  the  Faure  variation,  which  is  me- 
chanical rather  than  fundamental.  The  present 
Edison- Jungner  iron-nickel-alkali  accumulator  is 
the  first  combination  which  seems  to  be  a  real  rival 
of  the  lead  cell.  This  rivalry  seems  to  be  confined 
to  vehicle  and  traction  work,  and  the  alkaline  battery 
can  hardly  be  said  to  compete  at  all  in  the  heavy 
work  of  the  modern  power  plant. 

It  is  hard  to  say  just  why  progress  has  been  so 
slow.  Evidently  the  problem  is  a  difficult  one. 
Plante  was  exceedingly  lucky  in  finding  what  seems 

246 


ACCUMULATORS  IN  GENERAL  247 

to  be  the  best  of  all  combinations,  and  the  men  who 
have  developed  the  alkaline  accumulator  to  its  present 
mechanical  perfection  deserve  all  credit  for  that 
achievement.. 

Any  categorical  statement  as  to  what  can  or  can- 
not be  done  in  any  line  of  scientific  or  technical 
development  is  not  likely  to  hold  true  very  long.  In 
the  accumulator  problem  we  seem  to  have  made  ad- 
vances only  in  one  direction,  and  it  may  be  that  there 
is  some  fundamental  advantage  in  this  direction.  All 
our  successful  accumulators  have  plates  which  are 
very  slightly  soluble  in  the  electrolyte  used  with 
them.  There  is  a  host  of  other  possibilities.  The 
electrolyte  might  be  called  upon  to  furnish  a  large 
part  or  all  of  the  cell  energy.  Soluble  plates  might 
be  used.  The  charge  might  be  made  by  chemical 
means,  in  which  case  the  cell  would  cease  to  be  an 
accumulator  in  the  usual  sense  of  the  word. 

131.  As  far  as  our  experience  goes  we  can  describe 
the  ideal  accumulator  as  follows  :  — 

The  active  material  is  very  slightly  soluble  in  the  elec- 
trolyte. 

Current  is  carried  through  the  main  body  of  the  cell  by 
ions  different  from  those  which  pass  back  and  forth  at  the 
electrodes. 

These  two  conditions  seem  to  us  at  present  to  be 
necessary  ones  because  we  have  found  no  other  evi- 
dent way  of  insuring  mechanical  reversibility.  Even 


248  STORAGE  BATTERIES 

lead  sulphate  has  almost  too  great  solubility  in  sul- 
phuric acid,  for  negative  plates  lose  in  capacity  be- 
cause of  the  increase  in  the  size  of  lead  grain.  Lead 
peroxide  is  ideal  in  this  respect. 

A  reaction  must  be  selected  which  yields  a  large  amount 
of  energy  per  gram  equivalent  of  material  used.  While 
the  substances  used  in  the  lead  cell  are  unfortunate 
by  reason  of  their  high  equivalent  weights,  they  are 
fortunate  in  another  way.  Energy  is  obtained  not 
only  from  the  anode  reaction,  where  lead  goes  into 
solution,  but  also  from  the  PbO2  reaction.  Lead 
peroxide  is  one  of  the  electrodes  which  can  furnish 
energy  during  reduction. 

The  cell  must  have  a  low  internal  resistance,  otherwise 
its  efficiency  will  be  impaired.  Again  the  lead  cell  is 
a  fortunate  choice,  for  hardly  any  electrolyte  has  a 
lower  specific  resistance  than  30  %  sulphuric  acid,  and 
both  lead  and  lead  peroxide  are  good  conductors. 

The  chemical  reaction  must  be  perfectly  reversible.  The 
losses  in  the  lead  cell  are  almost  wholly  due  to  the 
production  of  gas. 

132.  The  first  efforts  toward  the  discovery  of  a 
cell  other  than  Planters  start  from  his  work  and  from 
his  point  of  view,  as  would  be  expected.  Peroxide 
of  lead  has  been  tried  with  most  of  the  metals  replac- 
ing lead  as  the  other  plate.  Zinc,  cadmium,  copper, 
bismuth,  etc.,  were  all  given  a  trial,  and  no  one  of 
them  has  proven  better  than  lead.  Then,  too,  the 


ACCUMULATORS  IN  GENERAL  249 

alkaline  combinations,  starting  with  the  Lalande- 
Chapeyron  type,  were  given  a  trial.  The  following 
may  be  mentioned :  — 

Copper /potassium  hydroxide/ silver  peroxide. 
Cadmium /potassium  hydroxide/copper  oxide. 
Zinc/potassium  hydroxide/ copper  oxide. 
Iron  oxide/potassium  hydroxide/manganese  dioxide. 
Iron  (?) /potassium  hydroxide/nickel  peroxide. 
Cobalt/potassium  hydroxide/ nickel  peroxide. 

133.  Until  recent  years  the  lead-sulphuric  acid 
cell  has  had  the  commercial  field  to  itself.  A  great 
many  suggested  combinations  were  tried,  but  no  one 
of  them  has  stood  the  test.  Usually  it  has  been  the 
mechanical  reversibility  which  has  been  at  fault, 
even  when  the  chemical  reaction  has  been  a  favorable 
one  and  quite  reversible. 

Lately  one  combination  has  been  developed  which 
bids  fair  to  make  a  place  for  itself  in  practical  serv- 
ice. It  is  already  a  success  as  far  as  all  tests  of 
reversibility,  mechanical  and  chemical,  are  concerned. 
This  is  the  iron/potassium  hydroxide/nickel  per- 
oxide cell,  as  developed  by  Edison  to  mechanical 
perfection  in  this  country.  Figure  96  shows  an 
assembled  cell.  The  cell  and  support  plates  are  made 
of  nickel  steel.  The  perforated  hollow  tubes  of  the 
positive  plate  (see  Figure  97)  contain  a  mixture  of 
metallic  nickel  and  nickel  oxide  before  development. 
After  development  the  active  material  is  perhaps 


250 


STORAGE  BATTERIES 


NiO2,  the  peroxide  of  nickel.  In  the  finely  perfo- 
rated flat  boxes  of  the  other  plate  (see  Figure  98)  is 
a  mixture  of  iron,  iron  oxide,  and  lampblack.  This 

is  the  negative  plate, 
and  on  charge  metallic 
iron  seems  to  be  formed 
in  part.  The  electro- 
lyte is  concentrated 
caustic  potash  solution. 
There  is  still  much 
to  be  learned  about  the 
fundamental  cell  re- 
action. The  simplest 
formula  is 

Ni02+Fe  =  NiO  +  FeO, 

and  this  is  a  fairly  close 
statement  though  not  an 
accurate  one.  This  for- 
mula indicates  one  inter- 
esting point.  The  elec- 
trolyte does  not  appear 
at  all.  And  it  is  quite 
true  that  the  change  in 

the  density  of  the  electrolyte,  from  complete  charge 
to  complete  discharge,  is  small.  There  is  a  slight 
change  of  concentration,  but  not  sufficient  to  be  of 
service  as  an  indication  of  the  condition  of  the  cell. 


FIG.  96.  — Edison  cell. 


ACCUMULATORS  IN   GENERAL 


251 


J 


It  is  of   course  perfectly  certain   that   there   are 

changes  of  concentration  of  the  electrolyte  in  the 
active  part  of  the  plates,  and  that  these 
changes  are  proportional  to  the  rate  at 
which  the  cell  is  working.  It  is  quite 
certain  that  the  effect  of  diffusion, 
which  has  been  called  on  so  often  in 
explanation  of  the  course  of  charge 
and  discharge  curves 
of  lead  cells,  plays  just 
as  important  part  in 
the  Edison  cell.  Until 
we  know  just  what  the 
fundamental  cell  reac- 
tion is,  we  cannot  fore- 
see just  how  great  the 
effect  of  change  in  the 

OH~  concentration  will  be. 

There  are  many  interesting  things 

about  the  curves  taken  on  this  type  of 

cell.    Figure  99  shows  discharge  curve 

to  a  low  voltage,  much  lower  than 

would  be  reached  in  practice.     The 

evident  two  stages  in  the  curve,  without  any  change 

in  the  distribution  of  active  material  to  account  for  it, 

may  mean  a  change  in  the  cell  reaction  at  that  point. 
This   particular   type   of   cell   has   the   following 

characteristics  at  25°  C.  :  — 


FIG.  97. —  Group 
of  Edison  posi- 
tive plates. 


FIG.  98.  —  Group 
of  Edison  nega- 
tive plates. 


252 


STORAGE  BATTERIES 


205 


135 


M.90 


170 


0.2 


HOURS 


FIG.  99.  —  Discharge  curve  of  Edison  cell. 


5C 


\ 


2 

HOURS 


FIG.  100.  —  Discharge  curves  of  standard  American  paste  type  at 
various  temperatures. 


ACCUMULATORS  IN  GENERAL 


253 


13 

V 

125 

\ 

\ 

5 

!• 

Ul 

105 

in 

\ 

\x 

^^^ 

^s, 

^>s 

\ 

\ 

x 

\ 

\ 

^  — 

^J0£. 

-^. 

^^ 

x 

^ 

s^ 

^ 

-^c 

^ 

~^ 

x 

^ 

\ 

^ 

x 

\ 

X 

\5'° 

x 

\ 

\ 

\ 

FIG.  101.  —  Discharge  curves  of  Edison  cell  at  various  temperatures. 

Weight  of  complete  cell,  19.25  Ibs. 

„  [ampere-hours,  225. 

Capacity 

y  [watt-hours,  248. 

Ampere-hours  per 
pound  of  cell,  11.3. 

Watt-hours  per 
pound  of  cell,  12.4. 

Ampere-hour  ef- 
ficiency, 82%. 

Watt-hour  effici- 
ency, 60%. 

An  examination 
of  the  temperature 
effect  shows  the  im- 
portant part  which 

FIG.  102.  —  Summary  showing  change  in    diffusion  plays   in 
ampere-hour  capacity  with  temperature.  .     . 

[Exide  and  Edison.]  the    Cell    activity. 


F256 


a. 

<I50 


i»  20°  30' 

TEMPERATURE(CENTIGRADE) 


40* 


50* 


254 


STORAGE  BATTERIES 


350 


300 


I 


The  curves  of  Figures  100  and  101  show  the  relative 
temperature  effects  on  a  standard  type  of  lead  cell 

and  on  an  Edison 
cell,  and  these  are 
summarized  in  the 
curves  of  Figures 
102  and  103. 

The  factors  which 
determine  the  prac- 
tical success  of  such 
a  cell  are  numerous. 
Without  any  inten- 
tion of  either  criti- 
cizing or  advertis- 

FIG.  103. --Change  in  watt-hour  capacity    ing   we  can  examine 
with  temperature.    [Exide  and  Edison.] 

the  general  charac- 
teristics of  some  present-day  types.  The  following 
table  gives  data  on  three  types  —  a  rather  heavy 
American  plate,  a  rather  light  European  type,  and 
the  regular  Edison  type  of  approximately  the  same 
watt-hour  capacity. 


IOC 


30° 


50° 


STANDARD 
AMKRICAN 

LIGHT 
EUROPEAN 

EDISON 

Watt-hours  per  pound     .     . 
Life  

8 
1 

12 

3 

12.5 

3 

Cost      

1 

1 

2? 

Watt-hour  efficiency  .     .     . 

75% 

80% 

60% 

APPENDIX 


The  General  Equation  for  the  Electromotive  Force  of  a  Cell 
in  Terms  of  the  Heat  of  the  Chemical  Reaction  and  the  Tem- 
perature Coefficient  of  the  Electromotive  Force 

Assume 

1.  The  law  of  the  conservation  of  energy. 

2.  The  second  law,  in  the  form 

work  done  dT 


heat  used  in  doing  it        T 

We  send  our  cell  through  the  following  cycle  :  — 

1.  At  temperature  T,  send  96,540  coulombs  through 
at  e  volts.     The  work  done  is  Fe  joules. 

Suppose  the  cell  cool  while  it  works.  It  will 
absorb  TFcalories  from  its  surroundings.  W—eF—  Q, 
Q  being  the  chemical  heat  of  reaction. 

2.  Raise  the  temperature  of  the  cell  to  T+  dT. 
This  will  take  P  calories.     The  electromotive  force 
is  changed  to  e  +  de. 

3.  Now  send  96,540  coulombs  through  in  the  oppo- 
site direction,  against  e  -}-  de  volts.     The  work  done 
is  F(e  +  de)  joules.     The  cell  heats  when  it  works 

255 


256  STORAGE  BATTERIES 

in  this  direction.  It  gives  out  W  +  dW  calories. 
TF+  dW  =  F(e  +  de)-[Q  +  dQ}. 

4.  Cool  the  cell  back  to  T.  We  get  back  our 
P  calories. 

dW  and  dQ  are  vanishingly  small.  They  can  be 
neglected,  since  d  T  is  an  infinitesimal  temperature 
difference. 

The  net  result  of  this  cycle  is  an  amount  of  avail- 
able work  Fde.  To  produce  this  amount  of  available 
work,  a  quantity  of  heat  Fe  —  Q  changed  its  temper- 
ature from  T-}-  dTto  T. 

Apply  the  second  law, 

Fde 


df 


Fe-Q       T 

Transforming, 


II 


Calculation  of  the  Electromotive  Force  of  a  Cell  in  Terms  of 
the  Solution  Pressure  at  the  Electrodes  and  the  Osmotic 
Pressure  in  the  Solution 

Assume  the  gas  law  to  hold  for  osmotic  pressures. 
pv  =  RT. 

p  =  osmotic  pressure. 
v  =  volume  of  a  gram-molecule. 
R  =  gas  constant. 
T '=  absolute  temperature. 


APPENDIX  257 

The  work  obtainable  by  a  change  in  concentration 
from  p1  to  p2  at  constant  temperature  is 


p  P2 

Solution  pressure  is  continually  balanced  at  the 
electrode  by  osmotic  pressure  and  work  done  is 
osmotic  work. 

p 

A=  ET  lne  --  where  P  is  solution  pressure,  p  is 

osmotic  pressure,  and  A  is  work  done  at  the  single 
electrode. 

We  are  calculating  in  gram-molecules.  For  a 
univalent  ion,  96,540  coulombs  will  pass  the  cell  with 
a  gram-molecule  ;  and  e,  the  electromotive  force,  will 
be  a  measure  of  A,  the  work  done  at  the  electrode. 

For  a  univalent  ion 

RT,     P 

e  =  -—lue-. 

F        p 

If  the  ion  which  maintains  equilibrium  is  bivalent, 
only  half  as  much  of  it  need  pass  the  electrode  to 
carry  the  96,540  coulombs,  and  if  it  is  ra-valent, 

—  -  as  much  will  be  enough. 
rath 

For  an  w-valent  ion  we  have 

.£. 

p 


258  STORAGE  BATTERIES 

At  the  other  electrode  we  have  a  precisely  similar 
equation  to  express  the  action,  but  here  the  ion 
passes  the  electrode  in  the  opposite  direction  and  e 
has  the  opposite  sign.  The  electromotive  force  of 
the  whole  cell  will  be  the  difference  of  the  two  single 
electromotive  forces. 


•,-,=... 

nF       pl       nF       pz 

RT 

—  —  -  is  constant  at  constant  temperature.     Its  nu- 

merical value  at  17°  C.  is 

8.31  x  290  x  2.303 


96,540 


0.0575. 


We  have  introduced  the  factor  2.303  which  changes 
natural  logarithms  to  common.  The  equation  as 
usually  applied  is 


0.0575 
e  = 


III 

Calculation  of  the  Concentration  of  the  Active  Ions  in  the  Lead 
Accumulator 

(1)   The  concentration  of  Pb++  ion. 
The  solubility  of  lead  sulphate  in  pure  water  is 
1.4  x!0~*  gm.-mol.  per  liter.     Assuming  complete 


APPENDIX  259 

dissociation  and  that  the  mass  law  holds  for  ionic 
equilibrium,  we  have 

Pb++  •  S04~  =(1.4  x  10~4)2  =  1.96  x  lO-8. 

Accumulator  acid  is  about  2  N  but  is  only  about 
50  %  dissociated.  In  this  acid  SO4  is  therefore 
1.0  N,  and  in  the  cell 

Pb++  =  2  x  10~8  gm.-mol.  per  liter. 

(2)  The  concentration  of  H+  ion. 

As  stated  above,  2  N  H2SO4  is  about  50  %  dis- 
sociated, the  concentration  of  H+  is  therefore  about 
2  gm.-mol.  per  liter. 

(3)  The  concentration  of  PbO2      ion. 
From  the  mass  law  :  — 

Pb02—  =  Pb++  .  (O-)2 
and  (H+)2  •  O      =  constant. 

Therefore, 

PK++ 
PbO2—  =  constant       +  ^ 

The  value  of  the  constant  can  be  calculated  by 
measurements  of  the  solubility  of  lead  hydroxide  in 
sodium  hydroxide  solution,  and  these  measurements 
are  within  the  range  of  analytical  attack.  In  Dola- 
zalek's  determination  the  sodium  hydroxide  was 
0.066  normal,  and  it  dissolved  Na2PbO2  to  a  Concen- 
tration of  0.00305  gm.-mol.  per  liter.  In  this 


260  STORAGE  BATTERIES 

solution  PbO2  was  therefore  about  .003  .ZVand  the 
remanent  alkali  contained  0.054  gm.-mol.  OH~  per 
liter. 

The  concentration  of  H+  in  this  solution  we  can 
calculate  with  the  aid  of  the  mass  law.     We  have 
H+  •  OH—  =  1.1  x  10-14 

from  measurements  on  water,  gas  cells,  etc. 

In  our  alkali  solution,  OH~  is  about  .05  normal. 

H+  is  therefore  about  2  x  10~13  N. 

The  lead  ion  concentration  in  the  alkali  we  need 
also.  In  pure  water,  lead  hydroxide  dissolves  to 
about  4  x  10~4  gm.-mol.  per  liter. 

We  have 

Pb++  •  (OH-)2  =  (4  x  10-4)3  =  6  x  10-n 
and  for  our  .  05  N  alkali 


Now  we  can  calculate  our  constant 


Pb++ 

K_  (3  x  IP"3)  •  (1.6  x  IP"51) 
2  x  10-8 


K=  3  x  10-46. 
From  this,  for  2  ^Vacid 


APPENDIX 

From  (1)     Pb++  =  2  x  KH. 
From  (2)        H+  =  2. 
(H+)4=16. 


261 


Finally     PbQ,- 


x 


Pb02—  =  4  x  10-53. 

This  is  the  concentration  of  the  PbO2      ion  in  the 
ordinary  lead  cell,  using  as  electrolyte  2  JVacid. 

IV 

Variation  in  Capacity  with  Volume  of  Electrolyte 

An  important  factor  in  the  design  of  a  storage  cell 
is  the  permissible  volume  of  the  electrolyte.     It  is 


123 


FIG.  104.  —  Variation  in  capacity  with  volume  of  electrolyte. 

A,  capacity  with  2000  cu.  cm.  of  electrolyte,  at  various  rates, 
a,  density  of  electrolyte  corresponding  to  A. 

B,  capacity  with  1100  cu.  cm.  of  electrolyte. 
ft,  density  corresponding  to  B. 


262 


STORAGE  BATTERIES 


quite  evident  from  general  considerations  that  in  a 
cell  containing  many  plates  and  little  electrolyte, 
the  latter  may  limit  capacity  by  becoming  so  dilute 
that  the  useful  working  voltage  is  soon  passed. 

Figure  104  shows  the  capacity  of  a  cell  and  the 
change  in  the  density  of  the  cell  electrolyte  at  differ- 
ent rates  of  discharge  and  with  different  volumes  of 
electrolyte  in  the  cell. 


The  Gas  given  off  from  the  Lead  Cell 

A  mixture  of  oxygen  and  hy- 
drogen is  given  off  from  a  lead 
accumulator  during  the  latter 
part  of  charge.  This  is  a  very 
explosive  gas  mixture,  and  in 
submarines  and  other  places 
where  batteries  are  closely  con- 
fined, ventilation  must  be  very 
carefully  looked  out  for. 

Figure  105  gives  a  diagram- 
matic picture  of  apparatus  which 
can  be  used  to  measure  the  rate 
at  which  gas  is  evolved  during 
charge  and  discharge.     The  gas 
escapes  through  the  narrow  cap- 
rate  of  evolution  of  gas.    illary,  and  the  gas  pressure  .  is 
measured  by  the  small  mercury  manometer. 


APPENDIX 


263 


Figure  106  gives  curves  of  a  test  on  the  rate  of 
gassing  of  paste  and  Plante  negative  plates  during 
charge  at  the  8-hr.  rate. 


500 


400 


a 

o 

g  30° 


200 


100 


THEORET  CAL  LIMIfT 


3456 
HOURS  OF  CHARGE 

FIG.  106.  —  Curves  showing  evolution  of  hydrogen  from  paste  and 
Plante  negative  plates  during  charge. 

VI 
Specific  Resistance 

Aluminium 3  x  10"6 

Lead 2  x  10~5 

Copper 1.7xlO-e 

Graphite  (about) 5  x  10~3 

Quartz. 3x10* 

30%  H2S04 1.4 

31%  HN03 .  1.3 

20%  HCI.   .;  .   : 1.3 


INDEX 


Accumulators,  general  considera- 
tions, 246. 

Acid  density  during  charge  and 
discharge,  44. 

Auxiliary  electrode,  use  of,  114. 

"Box"  negative,  237. 

Capacity,  116. 

and  acid  density,  134,  166. 

and  Faraday's  law,  117. 

arid  plate  thickness,  122,  123. 

and  temperature,  134. 

and  volume  of  electrolyte,  261. 

calculations,  124. 

change  in,  during  service,  193. 

curves,  theoretical,  119,  125. 

determined  by  end  voltage,  118. 
Car-lighting  systems,  240. 
Cementing  of  pastes,  196. 
Charge  curve,  at  various  rates,  102. 

complete,  98. 

first  part  of,  97. 

peculiarities,  99. 

various  types,  103. 
Charge  and  discharge,  94  et  seq. 
Charge     and     discharge     curves, 

individual  plates,  114. 

various  rates,  112,  113. 

various  types,  109. 
Charge    and    discharge    voltages 
(average)    at   various    rates, 
146. 

Charge  reaction,  41. 
Chemical  potential,  22. 
Commercial  types,  225. 
Current  density,  possible  changes 
at  high,  40. 


Daniell  cell,  19. 
Definitions  of  all  parts,  11. 
Deformation  (buckling,  etc.),  215. 
Densities  of  lead  compounds,  175. 
Diffusion    curves    and    recovery 

curves,  131. 
Diffusion,  general  discussion,  129. 

in  resting  plates,  129. 

Liebenow's  experiment,  129. 
Discharge  curve,  and  acid  density, 
107. 

at  various  rates,  120. 

first  part  of,  105. 

to  low  volleys,  110,  111. 

various  types,  121. 
Discharge  reaction,  49. 
Diseases  and  troubles,  207,  213. 

Edison  cell,  250. 

characteristics,  253. 

discharge     curves     at    various 

temperatures,  253. 
Efficiencies  at  various  rates,  144. 
Efficiency,  ampere-hour,  141. 

energy,  143. 
Electrical  energy,  25. 
Electrical  units,  13,  24,  25. 
Electro-chemical  unit,  21. 
Electrode,  standard,  82. 
Electrode  equilibrium,  86. 
Electrode  reactions,  81. 
Electrolytic  cell,  13. 
Electromotive  force,  22. 

and  acid  density,  77. 

theory,  256. 
Electrostatic    equilibrium    about 

an  electrode,  62. 
Energy  relations,  64. 


265 


266 


INDEX 


Faraday's  law,  11,  15. 
Formation  at  low  voltage,  188. 

Plants,  179  et  seq. 

rapid  Plante,  184. 

theory  of,  186. 
Forming  agents,  185. 

persistence  of,  191. 
Fundamental    energy    equations, 

67,  70,  255. 
Fundamental  reaction  formula,  40. 

Gas  evolved  from  lead  cell,  263. 
General     equation     for     electro- 
motive force,  255. 

Heat    of    dilution    of    sulphuric 
acid,  74. 

Impurities  and  local  discharge,  217. 

effect  of,  208,  212. 
Ionic    concentrations,  calculation 

of,  258. 

Ionic  theory,  33. 
Ion  reactions,  38. 
Ions,  12,  23,  30  et  seq. 

active,  during  charge  and  dis- 
charge, 50  et  seq. 

in  electrolyte,  48. 
"Iron-clad"  plate,  245. 

Lead  cell  reaction,  39  et  seq. 
Le  Blanc's  theory,  89. 
Liebenow's  theory,  90. 
Load  regulation,  229. 

Migration  of  ions,  36. 
Migration  velocities,  35. 

Non-lead  types,  249. 

Operation  of  batteries,  223. 
Osmotic  theory  of  galvanic  cells, 

256. 
Osmotic  work,  86. 

Paste    negatives,    change    during 
formation,  204. 


Paste  plates,  194. 

Paste  positives,  formation,  198. 

types,  237. 
Paste  recipes,  202. 
Physical  characteristics,  172. 
Plante  negatives,  192,  236. 
Primary  cells,  3. 

Reaction  velocity,  136. 
Recovery,  after  charge,  104. 

and  diffusion,  131. 

after  discharge,  107,  108. 

after  long  discharge,  133. 
Resistance,  27. 
Resistance  curves,  153  et  seq. 

factors  of,  155. 

of  sulphuric  acid  solutions,  149. 

specific,  148,  263. 

temperature  effect  during  activ- 
ity, 165. 

temperature  effect  on,  151. 
Restoring  capacity  of  negatives, 
214. 

Self-discharge  of  Plante  plates,  183. 
Shedding  of  active  material,  218. 
Short  circuits,  219. 
Solution  pressure  theory,  84. 
Stand-by  batteries,  232. 
Submarine  cells,  238. 
Sulphation,  216. 

and  internal  resistance,  157. 

Temperature  coefficient  of  electro- 
motive force,  72. 
Thermochemical  data,  66. 
Train-lighting  systems,  241. 

Vehicle  grids,  242. 
Vehicle  service,  241. 

Watt-hour  capacity,  137. 

at  various  temperatures,  139. 

diagrams,  138. 
Weight  capacity,  243,  254. 
Work  done  at  an  electrode,  64. 
Work,  osmotic,  83. 


'"THE  following  pages  contain  advertisements 
of  Macmillan  books  on  kindred  subjects 


The   Elements  of   Electrical   Transmission 

A   Text-book  for  Colleges  and  Technical  Schools 

BY  OLIN  JEROME  FERGUSON 

Associate  Professor  of  Electrical  Engineering  in  Union  College ; 
Mem.  A.I.E.E.;  Mem.  N.E.L.A.;  Mem.  Soc.  Prom.  Eng.  Ed. 


Cloth,  8vo,  vii+457  pages,  illustrated,  index,  diagrams,  $3.50  net 


In  preparing  the  material  for  this  book,  it  has  been  the  author's 
aim  to  present  those  things  which  should  be  grouped  and 
articulated  in  order  to  afford  an  elemental  study  of  the  very 
broad  subject  of  the  generation,  transmission,  distribution,  and 
utilization  of  power  by  electrical  processes.  The  title  of  the 
work  only  partially  covers  this  field,  but  was  chosen  with  a  view 
to  brevity  and  convenience.  In  fact,  our  language  needs  some 
modest  word  for  so  large  a  subject. 

Necessarily,  the  material  available  far  exceeds  the  capacity  of 
any  single  volume,  but  the  attempt  is  made  to  provide  a  working 
text-book  which  may  serve  as  a  more  or  less  complete  course, 
depending  upon  the  amount  of  time  available  for  the  subject. 
It  is  hoped  that  it  may  serve  as  an  outline  for  even  an  extended 
course,  supplemented  always  by  that  most  important  source,  the 
teacher.  Nevertheless,  the  book  will  probably  stand  or  fall  as 
an  elementary  text-book,  which  is  all  that  it  desires  to  be. 

Immediately  after  publication  this  book  was  adopted  as  a  re- 
quired text  for  use  with  a  large  class  of  students  at  the  Massa- 
chusetts Institute  of  Technology.  This  adoption  combined  with 
the  fact  that  Professor  Ferguson  was  fortunate  enough  to  secure 
the  valuable  criticism  and  advice  of  Dr.  Charles  P.  Steinmetz 
while  preparing  the  manuscript,  augurs  well  not  only  for  the  accu- 
racy of  the  book  but  also  for  its  availability  for  use  as  a  class  text. 


THE   MACMILLAN  COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


Revolving  Vectors 

With  Special  Application  to  Alternating  Current  Phenomena 

BY  GEORGE  W.   PATTERSON,  S.B.,   PH.D. 

Professor  of  Electrical  Engineering,  University  of  Michigan ; 
Member  Am.  Inst.  El.  Eng. ;  Member  Am.  Ph.  Soc.,  etc. 


Cloth,  8vo,  8g  pages,  index,  $1.00  net 


Earlier  writers  used  complex  quantities  to  represent  vector 
quantities  algebraically.  Dr.  Steinmetz  extended  the  application 
so  as  to  include  harmonic  quantities.  As  many  writers  on  elec- 
trical subjects  are  prone  to  confuse  vector  and  harmonic  quanti- 
ties, the  author  thinks  it  necessary  to  distinguish  these  two  uses 
of  complex  quantities,  and  for  that  purpose  he  starts  with  the 
vector  use  and  later  takes  up  the  harmonic  use.  In  addition,  sub- 
traction, and  certain  cases  of  multiplication  and  division,  correct 
results  are  obtained  by  treating  harmonic  quantities  as  vector 
quantities  ;  but  in  other  cases  of  multiplication  (such  as  multipli- 
cation of  e.  m.  f.  and  current  to  obtain  power)  and  division  (such 
as  dividing  power  by  e.  m.  f.  to  get  current),  incorrect  results  are 
obtained  unless  arbitrary  rules  of  multiplication  and  division  are 
introduced.  It  therefore  is  necessary  thoroughly  to  examine  the 
fundamentals  of  these  uses  of  complex  quantities,  and  to  deduce 
the  laws  of  addition,  subtraction,  multiplication,  and  division,  as 
applicable  to  vector  quantities  and  to  harmonic  quantities  whether 
simple  (electromotive  force  or  current)  or  compound  (power), 
and  also  to  such  non-harmonic  quantities  as  resistance,  capacity, 
inductance,  etc.,  in  connection  with  harmonic  quantities. 


THE   MACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


Magnetism  and  Electricity  for  Students 

BY  H.   E.   HADLEY,   B.Sc.    (Lond.) 

Associate  of  the  Royal  College  of  Science,  London  ;  Headmaster  of  the 
School  of  Science,  Kidderminster. 

579  pages,  $1-40  net 
The  Storage  Battery 

BY  AUGUSTUS  TREAD  WELL,  JR.,  E.E. 

Cloth,  257  pages,  #1.75 

An  aid  to  those  who  have  direct  charge  of  storage  batteries,  and 
to  those  who  are  interested  in  their  workings. 

An  Elementary  Book  on 

Electricity  and  Magnetism  and  Their  Applications 

BY  DUGALD  C.  JACKSON 
Professor  of  Engineering,  Massachusetts  Institute  of  Technology,  and 

JOHN  PRICE  JACKSON 

Professor  of  Electrical  Engineering,  Pennsylvania  State  College. 
New  York,  1907. 

Cloth,  I2mo,  482  pages,  $1.40  net 

The  author  begins  with  a  treatment  of  the  theoretical  considera- 
tions of  electricity  and  magnetism,  presenting  them  in  an  elemen- 
tary descriptive  manner.  The  theory  is  then  developed  and  the 
practical  applications  of  it  to  electric  machinery  are  given.  The 
book  will  serve  as  an  excellent  introduction  to  Engineering  courses. 
It  has  also  proved  to  be  valuable  in  the  hands  of  students  wishing 
to  make  a  hasty  review  of  the  whole  subject  at  the  close  of  their 
Electrical  Engineering  course. 


THE   MACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


The  Elements  of  Electrical  Engineering 

A  Text-book  for  Technical  Schools  and  Colleges 
BY  WILLIAM  SUDDARDS  FRANKLIN 

Professor  of  Physics  in  Lehigh  University,  and 

WILLIAM  ESTY 

Professor  in  Electrical  Engineering  in  Lehigh  University.    New  York,  1907. 

TWO  VOLUMES 

I.   Direct  Current  Machines  Cloth,  8w,  517  pages,  $4.50  net 

II.   Alternating  Currents  Cloth,  8vo,  378  pages,  $3.50  net 

Volume  I,  published  in  July,  1906,  has  become  in  the  two  years  that 
it  has  been  upon  the  market  one  of  the  most  widely  used  texts  on  the 
subject  now  published.  It  has  given  universal  satisfaction  as  a  treat- 
ment easily  understood  by  the  students,  and  yet  thorough  and  complete 
in  detail. 

Volume  II,  published  in  1908,  promises  to  be  equally  satisfactory. 

Electric  Waves 

An  Advanced  Treatise  on  Alternating-Current  History 
BY  WILLIAM  SUDDARDS  FRANKLIN 

Professor  of  Physics  in  Lehigh  University. 

315  pages,  $3.00  net 

"The  author  states  that  as  it  is  most  important  for  the  operating  en- 
gineer to  be  familiar  with  the  physics  of  machines,  the  object  of  this 
treatise  is  to  develop  the  physical  or  conceptual  aspects  of  wave  motion, 
that  is,  "  how  much  waves  wave,"  and  that,  with  the  exception  of  the 
theory  of  coupled  circuits  and  resonance,  it  is  believed  that  the  "  how 
much  "  aspect  of  the  subject  is  also  developed  to  an  extent  commensu- 
rate with  obtainable  data  and  the  results  derived  from  them.  While 
this  treatise  is  stated  to  be  complete  both  mathematically  and  physi- 
cally, as  far  as  it  goes,  the  student  is  referred  to  other  works  for  the 
more  elaborate  mathematical  developments."  —  Proceedings  of  the 
American  Society  of  Civil  Engineers. 


THE  iMACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


Applied  Electrochemistry 

BY  M.   DE  KAY  THOMPSON,   PH.D. 

Assistant  Professor  of  Electrochemistry  in  the  Massachusetts  Institute 
of  Technology 


Cloth,  8vo,  329  pages,  index,  $2.10  net 


This  book  was  written  to  supply  a  need  felt  by  the  author  in 
giving  a  course  of  lectures  on  Applied  Electrochemistry  in  the 
Massachusetts  Institute  of  Technology.  There  has  been  no  work 
in  English  covering  this  whole  field,  and  students  had  either  to 
rely  on  notes  or  refer  to  the  sources  from  which  this  book  is  com- 
piled. Neither  of  these  methods  of  study  is  satisfactory,  for  notes 
cannot  be  well  taken  in  a  subject  where  illustrations  are  as  impor- 
tant as  they  are  here ;  and  in  going  to  the  original  sources  too 
much  time  is  required  to  sift  out  the  essential  part.  It  is  believed 
that,  by  collecting  in  a  single  volume  the  material  that  would  be 
comprised  in  a  course  aiming  to  give  an  account  of  the  most  im- 
portant electrochemical  industries,  as  well  as  the  principal  applica- 
tions of  electrochemistry  in  the  laboratory,  it  will  be  possible  to 
teach  the  subject  much  more  satisfactorily. 

The  plan  adopted  in  this  book  has  been  to  discuss  each  subject 
from  the  theoretical  and  from  the  technical  point  of  view  sepa- 
rately. In  the  theoretical  part  a  knowledge  of  theoretical  chem- 
istry is  assumed. 

Full  references  to  the  original  sources  have  been  made,  so  that 
every  statement  can  be  easily  verified.  It  is  thought  that  this  will 
make  this  volume  usful  also  as  a  reference  book. 

An  appendix  has  been  added,  containing  the  more  important 
constants  that  are  needed  in  electrochemical  calculations. 


THE   MACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


Testing  of  Electro  Magnetic  Machinery 
and  Other  Apparatus 

BY  BERNARD   VICTOR  SWENSON,   E.E.,  M.E. 

of  the  University  of  Wisconsin,  and 

BUDD  FRANKENFIELD,  E.E. 

of  the  Nernst  Lamp  Company 

Volume  I  —  Direct  Currents  Cloth,  8vo,  420  pages,  $3.00  net 

Volume  II  —  Alternating  Currents    Cloth,  8vo,  324  pages,  $2.60  net 

It  is  a  book  which  can  be  thoroughly  recommended  to  all  students 
of  electrical  engineering  who  are  interested  in  the  design,  manufacture, 
or  use  of  dynamos  and  motors.  ...  A  distinct  and  valuable  feature 
of  the  book  is  the  list  of  references  at  the  beginning  of  each  test  to  the 
principal  text-books  and  papers  dealing  with  the  subject  of  the  test. 
The  book  is  well  illustrated,  and  there  is  a  useful  chapter  at  the  end  on 
commercial  shop  tests.  —  Nature. 

The  plan  of  arrangements  of  the  experiments  is  methodical  and  con- 
cise, and  it  is  followed  in  substantially  the  same  form  throughout  the 
ninety-six  exercises.  The  student  is  first  told  briefly  the  object  of  the 
experiment,  the  theory  upon  which  it  is  based,  and  the  method  to 
be  followed  in  obtaining  the  desired  data.  Diagrams  of  connections 
are  given  when  necessary  and  usually  a  number  of  references  to  per- 
manent and  periodical  literature  suggest  lines  of  profitable  side  reading 
and  aid  the  experimenter  in  forming  the  desirable  habit  of  consulting 
standard  text  outside  the  scope  of  the  laboratory  manual.  Before  per- 
forming the  experiment  the  student  also  studies  from  the  book  the  re- 
sults previously  obtained  from  standard  apparatus  by  more  experienced 
observers,  so  that  he  may  correctly  estimate  the  value  of  his  own  meas- 
urements. In  brief  form  are  listed  the  data  to  be  collected  from  the 
experiment  and  the  reader  is  cautioned  against  improper  use  of  the 
apparatus  under  test.  A  very  valuable  part  of  this  feature  of  the  in- 
structions consists  of  remarks  upon  empirical  design-constants,  many  of 
which  the  student  may  observe  or  measure  for  himself.  Certain  deduc- 
tions, also,  are  called  for  with  the  evident  purpose  of  showing  the  further 
practical  application  of  the  results  obtained.  —  Engineering  News. 


THE   MACMILLAN   COMPANY 

Publishers  64-66  Fifth  Avenue  New  York 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY 

Return  to  desk  from  which  borrowed. 
This  book  is  DUE  on  the  last  date  stamped  below. 
ENGlNFgRlNQ    pep 


27  J953 


ocr 


LD  21-100m-9,'48(B399sl6)476 


